Thank you for the kind responses. In answer to some of the questions:
DH and I have a big circle of friends and regularly spend time with them, either as a couple, as family or individually (meals out, dinner parties, drinks, walks, gigs, holidays together, hobbies etc) so DS should witness us modelling this behaviour.
I have long suspected that DS may be ND because of his struggles with interacting with his peers. DH and I have tried to give him simple 'rules' to follow but he seems to find it difficult to read social situations, and to do that things that seem to come naturally to other teens.
DS does get very upset over the issues he has which is why I worry so much about him. The main issue at the moment seems to be that he never gets invited to parties or social gatherings. He has lots of acquaintances though school and various hobbies, a few friends and has even had a couple of girlfriends, but always seems to get left out of the invite list even when there is no obvious reason for it.
I don't know what the right thing to do is. I really want to help DS but have reached a point where I no longer know how.