It can be hard to be in middle school. Does he have friends that are not IN his classes? That can be an awkward place to be. I can remember starting middle school and feeling weird and alone because NONE of my friends from elementary was in any of my classes. Not one. I slowly made friends with some of the people in my classes, but then those kids didn't necessarily stay in all my classes the following year. It can be tough if that sort of thing happens, and there isn't much control your son has over that.
Our son (11) started middle school this year, and I was very worried about him. He had been in private school (2 different ones starting in Kindergarten) from the beginning, until we just couldn't afford it anymore. He always makes friends wherever he goes, and is very social, and a sweet kid. But I noticed that this year it is alot like you have described. He comes home and mentions this kid or that kid, but he didn't do ANY outside of school activities with ANY of them. He did have a "sleep over" with one boy, who's mom/dad I met by chance up at my son's karate school (the other boy is a student also, but a different rank so they are not in the same karate class). But that outside of school relationship did not blossom or continue. It gets much harder for parents to talk to each other at this age, and so harder for us to intervene to ensure they get together outside of school. It's really up to the kids. My son has a lot of friend/acquaintances outside of school, and 2 pretty good friends (one from his old private school, and a girl from church that went through confirmation classes with him). I am hoping that next year he will find his niche and become good friends with one or two boys at school that I can get to know, that he can go forward in school with.
Maybe none of your son's friends happened to be in the class that had the party you were at? I know my son had classes last year where he didn't really have "friends"... and a lot of the kids already had cliques or peer groups formed from elementary, that he wasn't part of and it can take a while to break into those kinds of groups. Especially at a large school. My son's middle school (6th, 7th, 8th) has around 1200 kids! And they actually are quite limited in their free/socializing time. They are very structured, to hear my son talk about it...
Keep an eye out, ask him if he wants to have a friend over or a friend that wants to see a movie with him (something that is just coming out)... and then have him call and set it up. Maybe he just needs help setting up time outside of school. I don't see any need to "push"... but give him opportunities. If he wants to act on them, he will.
hth
:)