@helgarr as a former teacher who has dealt with this sort of scenario many times, I would suggest you are proactive in your dealings with the school. Ask to meet the relevant staff dealing with this, discuss with them your concerns about your son's capacity to understand the impact of his actions and a potential autism diagnosis, and go from there. Don't wait for them to contact you. You need to show you're taking this seriously.
I would hope that the school will recognise that your son has some additional needs that should be taken into account. If he has been at the school since he was 11 then I can't imagine this won't have been flagged repeatedly before. You say he makes consistent poor choices so I'm sure there will be a record of behaviour kept at school to demonstrate that this is part of a pattern and not a one off.
A sincere and heartfelt apology is a good place to start but you need to be led by what the girl in this scenario wants. She may not want that and you and your son will need to respect that. She needs to be able to voice what she wants.
I completely agree by the way that this kind of behaviour from male students towards female is totally unacceptable and punishment should be meted out. However, it does need to be recognised that these kinds of misogynistic comments come from a wider societal culture where misogyny is everywhere, is constantly minimised and even encouraged, and there are very little to any consequences for men who are violent verbally or physically towards women. So where are boys supposed to learn that this isn't ok? It's widely socially acceptable to make horrendously sexist comments with no consequence - even on TV, radio, in newspaper articles - look at the latest furore with Jeremy Clarkson and what he said about Meghan Markle. If their favourite TV presenter says something horrific about a woman and everyone just laughs, how do boys know it's wrong to say that? These sorts of comments don't exist in a vacuum and rather than seeking to blame and shame this young man, there needs to be an understanding that society is failing young men and women by continuing to accept misogyny as a norm.
OP it sounds like you're doing your best in a difficult situation. It's a very complex world for young people to navigate out there right now.