I think there are times most of us do not want anyone in our business, like when setting up camp (esp. if you have to unhitch, level, etc.). We don't appreciate folks inviting themselves to join us at supper or other activities. That said, MOST of the time I don't mind company and have enjoyed meeting new people when camping.
The child may or may not have Asperger's, may or may not be an only child and may be lonely. It does sound like his parents weren't paying much attention to him...then again maybe someone was keeping an eye on his visit.
Was his behavior strange? Not really. We didn't allow our girls to pester anyone, including such visits, looking in vehicles or camper/tent/rv. But that's just the way we raised our children. Was the OP out of line or rude...maybe. But if that youngster had come over after they'd set up and were relaxing or with one of his parents, then maybe the visit would have gone much better.
I would not want to have someone else's child tagging all over with us the whole time...it would really have me wondering what on earth the child's parents were doing???
DH is especially good at these situations. He'll engage the visitor and give them something to do to help...he's really very friendly and great at this stuff, more so than I. OTOH, we know we've asked people questions about their rig or toads and so on. But we don't bother them during meals or setting up and try not to overstay such visits.
It's hard sometimes, but sometimes we just have to step back, think what's really going on (with a curious, very active child) and be patient...after all he's just a little boy:)