Thanks for the advice Senorita. A while ago I asked if he'd like to invite anyone from nursery and he said 'no' as if I were completely mad to even suggest it!
I think he is slow socially compared to other children. Recently, he seems to be paying attention a bit more to other children, copying what they are doing and even asking names and ages at playparks etc, but that's about as far as it goes. Other friends we have with children visit occassionally and sometimes he warms up and plays with them a bit but usually not much. Maybe I'll ask about inviting someone from nursery again and see how he feels.
A while ago I mentioned the bullying concern at nursery and they said they had not noticed it but said that he is more of an observer than a joiner-in. I was a bit like this as a child where large groups were concerned and didn't really 'click' into group type friendships until I was about 6/7. I always had cousins to play with when I was younger but he doesn't have this. I think my concerns are that if he is quiet and withdrawn, he may be picked on when he starts school - especially as a boy. And this might start a vicius circle.
Activity-wise, he likes most things - running around outdoors, messy play, reading, etc
I do have a niggling concern that he may possibly be on the autistic spectrum (but only just) because of some of his behaviour, but this seems to come and go - and so it's all just very confusing!! Again, mentioned this at nursery and two members of staff pulled faces that expressed a lot of doubt about this but said they weren't experts and another said 'possibly but it would only be very borderline'.
Trying to do the best thing for children is HARD...