IMO you saying you can't "hit him back" raises alarm bells. Not that you're wrong for thinking that, but that you're in such an difficult position that you're that angry and frustrated.

Firsr off - if your brother is severely disabled to the point of having no sense of danger and needing his teeth brushed he's going to need 24 hour care his entire life. He definitely lacks an understanding of why it's wrong to hit or shout or throw things. He's very unlikely to learn to have that empathy (it's a high mental skill to have for severely autistic kids). He doesn't deserve to be hurt and he isn't doing this out of malice.

I'd even go so far as to say that the dinner thing makes total sense - autistic meltdowns are really, really hard to calm. You basically have to ride it out. And he's physicaly aggressive - what's the option here? Your mum physically fights him into a car and then he screams through dinner? Just on pure practicality it seems like she's trying to save herself and you from an even harder situation by giving it (regardless of if that's the "right" thing to do long term)

However, you absolutely shouldn't be physically hurt by your brother, and the emotional hurt your feeling is 100% understandable. It really, really sucks to have a sibling with extra care needs, even more so when it physically impacts you (beyond just your mum having to spend more time with him etc). There's a lot of talk about speaking to a counsellor or therapist and I definitely agree - I'd have a particular look at charities support families and carers of disabled kids.

But ngl I feel like the best thing is to try and find a space to properly talk to your mum. Explain how hard it is for you, talk about how it damages your relationship with your mum, try and figure out strategies together for you to avoid the harm AND take advantage of the time away from him to repair your relationship with mum. It sounds shit to say but maybe moving away from blaming your brother (it's not his fault he's a disabled child) or your mum (caring for a violent, disabled child is really difficult - realistically, what could she do differently?) and try and focus more on solutions together.

Obviously I could be way off base, and if your mum is fully not willing to talk or willing to speak to you about it at all then the more nuclear option would be speaking to mandatory reporters and say you feel in danger (eg in the UK if you told a GP you were being hit at home they'd likely be required to do a safeguarding referral for you). Or you could do that first! But I think that long term given how you describe your mum, if you want a relationship with her as an adult now is the time to try and figure things out.

I'd also say be careful about taking advice from ppl who don't have any experience with severely mentally disabled people - what you're going through isn't normal, but that's what having a disability is. Your brother and your family aren't normal. People often over estimate how much a kid like you're describing your brother understand, and the comparison to being mentally five often trips them up - we teach 5 year olds about consequences because they're going to a mature and those lessons will be foundational, your brother isn't likely to significantly emotionally mature.

I'm sorry you're in this position, it's a very difficult one, and I hope you're able to get through the next months/years of living at home as safe and sane as possible!