He broke his Gran's wrist? Bloody hell! I will say that boys of this age sometimes don't have the self-awareness that they are not babies but almost grown men in terms of size and strength, but he should not be "tackling" his Gran! What consequences have ensued? If she'll allow it, he should at the very least be looking after her, doing her housework, making her bed, bringing her tea. etc.
But what is happening about his ASD? Does he have help to manage it? Does he have ADHD as well? - it sounds as if that is a possibility.
I'd start with the GP for a referral, and also email the SENDCO at school to see what they have put in place and what help they can offer.
I don't know enough about what support is available but there are lots of experienced people here and hopefully they can help.
It's probably not just his personality, but his condition which makes him difficult to live with- if the right support was in place, which might include medication and a structured behaviour plan, he might become easier to deal with.
It's worrying that his gran has already suffered physically, it sounds as if his younger sibling could be at risk. You need support to help him, it sounds totally overwhelming.