At seven, your son is starting to compare himself to others. I think it's important that he know that he's different, and it's not his fault; but I think it's equally important that you stress that all those strengths he has are so much more important than the weaknesses; and that being different doesn't make him inferior.
There's a balance here: Eventually, you begin to understand that you're different; and if you don't know it's just a PDD, you begin to worry that maybe you're inferior somehow--a geek, a bad child, or even stupid. But whether or not your son has started to worry about this yet is not clear: His enthusiastic participation in class seems to show that he has a good self-concept (and well he should, if he's getting 102s on tests!).
Maybe it's like telling kids how babies are made: You start out with simple things, and eventually, as they age, you reveal more, so by 13 or so they know all the details. At this point, all he really needs to know is that some things are easy for him, and some things are hard; and when things are hard for you, people help you with them.
But as he gets older, he'll begin to be more observant, and compare himself to others. If he talks about being different; if he talks about other kids not liking him; then you can take that as a sign that he needs to know that there's nothing wrong with him--that he's just different, and that he'll always be the kid you love, with a lot of wonderful skills and talents.
By the time he's 11 or 12, he should know what PDD-NOS means--that it's harder for him to learn to socialize, etc.--and that it's not going to hold him back from doing what he wants to do, or from being happy.
After all, if most kids were like your son, it'd be the "normal" kids who were rated as disabled--because not everybody can get 102s on tests, right?