Are you getting services for David? A lot of times, it is hard for a child with Asperger's to relate to other kids (or vice versa), but they do an excellent job relating to other children with Asperger's. Also, it might be worth getting him involved in activities he LOVES, and he's more likely to make friends who share these same interests.
Personally, I don't agree that it should be put into his IEP that he gets invited to all birthday parties. I have a daughter with Asperger's, and I would never want somebody forced to invite her. I think that's highly inappropriate and would build resentment. What would happen later in life? How would the adult cope then? The solution is to do behavior therapy and social skill classes. Get his abilities up! He might see that with the effort, those invites will begin pouring in! This has worked wonders for my daughter. She used to get shunned from playdates, etc. It really motivated her to improve her skills. It's not always easy, and she does relapse at times to what's comfortable for her, but she sees the difference in her efforts with more friends, playdates, and invites!
ETA: I will change my comment to I don't think a parent should be required to invite an entire class to a birthday party in elementary school. For one thing, the kids are beginning to separate by gender. I did invite the whole class to my daughter's party (by choice) and the boys sat on one side of the room, the girls sat on the other side of the room. For another thing, class sizes are huge (and will be getting even bigger). This can limit the type of party or make having a party too cost prohibitive.
If my daughter were to find out about a party she wasn't invited to, I'd have to help her through that issue. It's not pleasant, but nobody gets invited to everything! It's something she (and everyone else) has to learn to cope with.
I do LOVE what another person posted about hosting your own little gatherings, just doing fun things! That's great fun and will definitely help the invites come in too!
Aspies can be great! I love mine to pieces and am beyond proud of her!