It would probably be a good idea to discuss this when you are both feeling calm and regulated to try to troubleshoot.
It might also be a good idea to look into whether there is any professional help available. Even if he’s not generally an angry person and it doesn’t seem like he needs anger management, some kind of therapy might help him to identify what causes these reactions and what might work for him to cope.
Also consider whether you need someone to talk to (possibly someone who doesn’t know either of you if you are concerned about the stigma of friends or family knowing that he did become physical during a meltdown).
Find the cause/s
It sounds like this is something you both already try to do, which is great. Knowing what’s causing them can sometimes make it easier to avoid them happening, or to prevent overload or distress from escalating to the point of violence. Being able to pre-empt that certain situations or problems might cause a meltdown means you can also put coping mechanisms in place. You might need to break down the cause/s, as well (for example, if your partner tends to meltdown after going out, think about what kind of outings tend to be too much and why that might be - length of time, the specific place/s he’s been, the travel method). Then you can think of things that might mitigate those difficulties, or be prepared for the fact he is likely to have a meltdown at some point if he’s unable to regulate/calm down.
Examine both of your behaviour during meltdowns
Does he tend to isolate himself or act not-towards you, or when he gets very angry does he seek you out? It sounds like it’s probably the former because you mentioned that he doesn’t target you or your things. However, if it is ever the latter, then that might suggest you do need to make sure you are away from him and safe, to prevent him being able to approach you if he might become violent.
Do you tend to try to help? Talk to him, calm him down, ask what’s wrong? If so, this would be perfectly natural - it’s normal and kind to worry about the people we love when they’re distressed, and to want to find out what’s causing it and to try to help. But sometimes having someone talking to, touching, or reacting to a meltdown can make things worse. Think of it as a state where you aren’t able to properly process and act rationally, so people trying to ask questions or reassure us can be overwhelming in itself, might cause more confusion or frustration (if we aren’t in a fit state to be able to reply properly or to act on what we’re being encouraged to do).
This does not mean that you are to blame for any violence enacted upon you, so please do not feel bad or blame yourself. But it might help you to react in a more helpful way if you are doing anything that exacerbates things.
Figure out the early signs
It’s not always possible for an autistic person (or the people around them) to recognise that certain behaviours might be signs of overload or distress, and sometimes the switch from appearing perfectly fine to being in a state of shutdown or meltdown can be so rapid and sharp it seems like it’s come out of nowhere. But by paying attention to little changes in behaviour or mood it might be possible to start to recognise hints that something is going on. If either of you start to recognise that he starts to become less able to articulate, or starts to huff or sigh or roll his eyes when he’s becoming overwhelmed or agitated, it might mean that he knows to make an active decision to self-regulate, or that might be a good time to offer a hug if that helps him. Being able to identify when I’m struggling, or having someone who loves me kindly point out that they’re noticing that I’m getting antsy and checking if I’m dehydrated or want to go off and sit in the quiet and do something that helps me to re-regulate can be hugely helpful. It might also enable you to quietly remove any valuables that might get broken, and yourself if need be, or to implement a strategy before he reaches the point of losing control (like getting out a box of things he can throw safely or something he can hit like cusions or a punchbag).
Have a strategy in place for the meltdown
It might not be possible to pre-empt, and once a meltdown has started it can be hard to think and act rationally, so it’s not always going to work, but, as mentioned above, you could try to come up with a box of things that it is safe for him to throw or to hit. These things can either be always somewhere visible where he might hopefully use them if it’s been pre-discussed and he’s aware of what they’re for, or you can get them out when he starts to meltdown and leave them where he can see them if it’s safe for you to do so.
Part of the plan might also mean you removing yourself from the area he’s melting down in if it is potentially going to result in you coming to harm. You don’t have to risk any injury to yourself. Meltdowns can be exacerbated or drawn out if things occur while they’re happening that cause additional distress, so lashing out and hurting you or you being frightened by him throwing things is potentially going to make it harder for him to calm down anyway - your priority should be your own safety anyway, but it might help to think of it as something that benefits both of you. Discuss this in advance to make it clear that you are leaving the area for safety and not because you are upset or angry with him and leaving in a negative way, but ultimately your safety comes before his feelings so if it is a bit upsetting to him to be left while he’s melting down, that’s something you can both address afterwards when he’s calm.
Meltdown/Shutdown Resource
You could both have a look at this resource to see if there is anything that resonates behaviourwise, or that sparks ideas for things that you experience that aren’t included, and whether any suggestions for things that help/don’t help are useful (or spark other ideas for things that would work for you both).
It’s quite generalised (so for example, too much socialisation is broad, and it might be more useful to someone who recognises that is an issue to try to narrow down the things that make it more or less tolerable - the specific people being socialised with, the amount of people, the amount of conversation/interaction involved (cinema trips might be better than a chat in the pub), the venue, the amount of time spent socialising, accrued socialisation over an extended period of time (five hours one evening might be tolerable if there is some time to regulate beforehand and time to recuperate the next day, but 1.5 hours per evening over the course of three evenings might be too much).
For some people, it might be that there is no major thing that causes a meltdown, but a build up of tiny things, and avoiding meltdowns is more about being aware of how many little things are happening. Those things might not actually seem like much of an issue by themselves, so sometimes it can even be about trying to figure out what could be contibuting rather than having to identify something is definitely contributing (if that makes sense?); if in one day I got up and had no milk left to have my routine breakfast but managed to get over it fairly quickly, then went to a shop and got something I was excited to get, then went to an event that I enjoyed, then came home on the tube still in a good mood, then ordered something to eat and it was delayed, then my t-shirt got caught on a handle as I walked past it and I had a huge, disproportionate meltdown, none of the things that happened were necessarily to blame, but all might have contributed - disappointment, sensory overload and crowds, more crowds and having to use public transport, gradually getting kind of overstimulated but doing things I enjoy and that making it bearable and making it harder to recognise, and eventually something tips me over the edge. So perhaps the next time I might need to plan to only do one thing in the day that requires going to a crowded place, or I need to remember my earplugs on the tube, or I need to make sure I eat earlier in the day or order food a bit earlier to allow for delays. And recognising it’s a build up of lots of little things that by themselves I was able to cope with helps it seem less shameful than believing that I flipped out over my t-shirt catching on a door handle.
And being aware that this can be the case can help with being more aware and mindful about the fact that the little things add up, it might be better to limit activities (take some things out of your bucket), or that you might need to actively make sure that you’re regulated, hydrated, have eaten, etc. to avoid your bucket overflowing.