I think that if he had feelings about never having a friend over, then probably it would actually have made him feel better to finally have a friend over? He certainly seemed to be loving it when it happened. No idea whether or not he used to have other friends over or not - whether it was a blanket no-playdates-at-home policy, or just DS who didn't get invited over.
And I very much doubt that he was part of the decision not to have people over. During the infamous "seeing Adam through the window" incident, Adam was literally jumping up and down with excitement to see DS waving from the car.
This was one of the mystifying things about the whole situation, to be honest. If it had been a more one-sided friendship, then I could kind of understand it (though I'd still think the parents were a bit rude not to reciprocate once or twice). But Adam and DS really had a great time together - I saw enough of them at our house to see that! - and hung out every chance they got at school. Adam was an awesome kid, and his parents were really nice, too: friendly, good fun, seemed very confident and cool. (We had the money, but they had the cool!) We got on well, chatted at the school pick up, etc. It genuinely baffled me that they just never ever invited DS for a playdate. It wasn't until I read a Mumsnet thread like this one, years later, that I wondered whether it was a big house/small house thing, purely because I just couldn't think of another explanation.
Reading this back, though, I'm beginning to wonder about another possible reason. Maybe it wasn't the house at all. Maybe our family just weren't very cool?! I mean, I'm an accountant, fgs, and it doesn't get much less cool than that. So I guess that's a possibility. Although they didn't seem snobby in that way, and that school wasn't exactly overrun with arty parents, so if they were waiting for that then they'd be waiting a long time. Who knows? The whole thing is still just a mystery to me, really.
Sorry, just thinking aloud there!