J.W.
I have to ask is he autism spectrum or something similar?
That could have been perfectly normal behavior for my 13 year old if I wasn't constantly correcting him. He just doesn't intuitively get that behavior is not normal for a teenager. He doesn't mean anything by it but he has no idea it is creepy. He still doesn't understand why it is creepy but accepts it is and doesn't act that way.
Okay Andy doesn't slap people on the butt, at all, still....If he had not been taught his whole life what is or is not appropriate he would be exactly as you described.
If he isn't diagnosed you may want to suggest he get tested because what you described is a child who doesn't see social boundaries.
Oh, just want to say you are not being overprotective. I know my son and I know why he acts the way he does and I tell people what is going on after I correct him when something new comes up. If I saw a child acting like my son and no one corrected him or explained it I would think what a creepy kid myself, ya know?