Do hope your stepson has received more christmas cards by now, stressyhead. I don't have any clear suggestions to make about your stepson being a loner at school. I know my son, aged 9 has gone through phases of saying he has few or no friends. His friendships this year especially, seem very tied in with what hobbies he follows ie when he's into a beyblade phase, he'll gravitate towards boys who like beyblades. His two best friends like pokemon just as he does, surprise surprise! He's not one for football, so does not mix much with the football crowd. But this could all change in two months time.
So, if your stepson has any hobbies like the above, you might consider encouraging them, which might mean forking out some cash, if possible. Then if he starts getting in with some boys who like the same things, you can suggest he invites one back for tea and take it from there.
I think you have to keep lines of communication open with him, make sure he feels happy to talk to you if something is bothering him, but if you feel he really isn't fussed about not having many friends at school, then as others have said, take your cue from him and relax.
I was a bit of a loner at school. I had enough friends to see out of school, and I really didn't want to be part of a crowd. It didn't bother me at all.