Lonely 26 year old son - what as parents can we do?

Hello.

We have a 26 year old son who is doing well academically (studying for PhD) but apart from this is struggling.

He spends the majority of his time in a studio in the garden, either reading or studying (his degree is part time distance learning so he rarely goes to the university).  This has been the situation for the last eighteen months. 

Despite trying he does not have a friend. Not one. He has been in two relationships in recent years but both girls finished with him due to his OCD challenges. He does not like pubs so that's out as he doesn't drink alcohol. He has no friends from school or university whilst doing his under grad and post grad.

He is a really nice person even though as parents we are biased!

it is really saddening to see that he has nobody apart from his mum and I to talk with as there is no family relatives. No friends. No one. 

Any suggestions as to how we could help him to at least have a chat with others that may lead to friendship such as clubs or associations he could join?

Thanks in advance.