if your son has a LSA, talk to the school and see if she can go out into the playground at break times and encourage interactive play, she could also keep an eye on some of the 'friends' too. At the school I worked at we used to hold small social awareness groups where we could act out a scenario and let the children decide what they thought was the correct response/interaction and then correct them if they got it 'wrong' as this was in small groups with invited classmates, it helped with their social integration.