This is something you have to let the kids handle, if you have their mom force them to "be nice", it will only make them resent your son more. They are upset about being chastised, and since they cant do anything to you, theyre taking it out on your son. Theyre only children, theyll either get over it and your son will have his friends back or they will keep it going, let the kids work it out. Maybe have a little talk with them yourself, dont go through their mom, and just explain that you know that you upset them when you chastised them, but you couldnt let them talk that way, and your son really misses them and he didnt do anything to them, so it would be nice if they would continue to play with him. Just explain to your son that he didnt do anything wrong, they are just upset and dont want to be played with, not everyone is always going to be nice and do the right thing, so he just needs to find new friends to play with. Take him to the park and invite over friends from school. If there are a few other kids in the neighborhood, invite them over. Give the other kids time to realize their mistake.