Are there other classes your son could go into? My daughter changed classes and has made some great friends.
As a pp said make an appointment with the teacher and look at strategies to help your son.
If that doesn't work then I would look at changing schools.

Do you do play dates with other kids from school? It can help foster a one on one relationship with the other kids.

What about scouts? What about kids youth groups? children can form friendships in these places and even a friend that's not at school with you is a good start! Even better if they make friendships and discover the child goes to their school!

First of all, a big hug to you and your son. My son is an outlier at school due to behavioral issues. We do A LOT of extracurriculars. He does on or 2 sports per season and tries other clubs like dance, art, fighting, boradcast, cooking, music, foreign language... these outside school activities have allowed him to try on different versions of himself and make friends. The difficult part is it's tougher to arrange hangouts than just picking an extra kid up at school, but it helped him socialize in peer settings so school isn't the nightmare it was. Until he could manage his own behaviours we had practice friends (mostly kids of my old friends and cousins we visit a few times a year). These playdates helped him understand social situations in a comfort zone, with me nearby and a few kids who had to be nice for a few hours. He's got a long list of good friends now, and while he still doesn't fit in as well at school, the behaviors have all but stopped

Seriously sports is key for a boy to make good friends in primary school, put him in alll the sports!

My son was in your sons predicament. it is hard as a parent watching them go through it. Haven you spoken to your classroom teacher. Are they aware of what is going on and the dynamics of the group? I would go in and and say my son has been acting out lately and he has finally told us that this is the reason, that he has no friends and finding it difficult. Make it more about your kid then blaming the other kids and you should be able to com up eith a plan to help him. Is he is the same class as his former friends, if so then consider moving class. Also join him up to clubs and things outside of school. They are a good way to meet new kids and make new friends and build up self esteem. Good luck.

What is at the core of the acting out behaviour? Maybe the resolution of that means other kids want to be friends with him. Also, does he have friends outside of school? Is he involved sports? Maybe a team sport would help him bond with his peers.