I've known quite a few bullied teens. (I home educate, and some of them leave school to escape the bullies.) One thing which can help is socialising with different age groups, so there's less likely to be any bullying. This may give your son the confidence boost he needs so he can start trusting people and feeling safe.
He's now old enough that there would be no issue with him joining hobby or voluntary groups which are mostly attended by adults significantly older than himself. The people there are likely to be delighted to find a young person wanting to join them. He can have a different sort of relationship with them.
Even good-quality teenaged friendships tend to involve more work than friendships with older people. Adolescence is a turbulent time. Sometimes teenaged friends will have their own stuff to deal with, so much so that they may not have the energy to be there for your son when he needs them. They're all figuring out who they are, worrying about whether they'll get a good job or ever find a boyfriend/girlfriend or whether they should take drugs or move out of the family home. Of course, older adults have problems too, but the intensity is less. They can be stable and dependable.
Adults will have found some of the answers. They may be able to help him through and show him the light at the end of the tunnel.
Likewise, being with younger kids will feel safer too. They'll idolise an 18yo and feel lucky he will give them the time of day. They won't criticise him. He'll be able to feel like the competent one who has his stuff together, who can be helpful and kind and be appreciated. So maybe he would like to volunteer with Scouts or a reading programme or spend time with younger cousins.