Does he have a decent haircut and presentable clothes?
Any disgusting habits? Throat clearing etc.
Maybe explain about conversations - taking turns. Being interested in what people are saying rather than just waiting for your turn to say something. Does he speak at a normal pace? Being cheerful or trying to sound cheerful is probably more appealing
Does he behave badly to girls - I mean more is he awkward or saying inappropriate things.
Does he stand over close to people?
Does he have an obsessive interest that he bores people to tears with?
Can you have a word with the teacher to see if she has any suggestions?
I think he is more likely to make friends if he's involved in some activity. Church based activities tend to be more inclusive. Or a sports club. Or learning coding or playing chess.
Is he interacting with children who are possibles? If he is trying to angle his way into the most popular cliques he is probably going to be disappointed.
I was not popular as a child so I do sympathise. I was a very plain teenager who was swotty. When I got to university I had grown out my dreadful haircut, got contact lenses, had my teeth fixed, waxed my legs (after years of my mother insisting that once you started shaving you had to keep it up so I had yeti like legs) had my dreadful acne brought under control by a dermatologist and stopped my mother knitting me jerseys. It made me see people as fairly shallow because I was the same person but suddenly men were dropping books and racing up to chat to me.