I'm sorry, I didn't have time to read all the replies, but I wanted to share my story with you.
My son is 5.5yo. He was always very timid around most people and wanted me or his dad to be with him all the time. At playgroups, he'd want me to move with him from activity to activity and he only did some interacting with the other kids. As for adults, if anyone even looked at him (never mind tried to talk to him) he would hide behind me or look away from them. I'm sure there were some people who thought he was rude, but I never heard any direct comments. As he grew older, I would discuss it with him and tell him it was OK to just say Hi to the adults, but that he didn't have to talk with them if he wasn't comfortable. I didn't push him. Even with close family members, he was pretty reserved and wouldn't give hugs or kisses, unless he wanted to (I never pushed him into that either). Talking with them was also difficult for him. A lot of it may have something to do with the fact that we were living 250 miles away from all family and I felt kind of isolated (plus we saw them only a few times a year). I believe that his personality plus the isolation played a part in his behavior.
Anyway, when he was 4.5yo we moved close to family and old friends. We used to live in a single-family house, set back from everyone else (kind of isolated) and we moved into a condo. Well, the very first week we were here, he was outside with me and he was saying Hi to everyone he saw - children & adults. It's as if he broke out of a shell. He's still a bit reserved around some people and I respect his feelings on that. I still don't push him to interact more than he wants to.
I do think this would have happened (more slowly) even if we hadn't moved. The move made me feel better (now I'm only 10 miles from my family) and I think my son felt my internal sigh of relief that I was home again.
I believe that honoring my son's feelings is much more important than what other people think. They're adults, they'll get over it. I want my little boy to grow up knowing that his feelings are important, even if others don't think so.
I hope this helps. Once again, I'm sorry I didn't read all the replies. Time is short today.