I think you have hit the nail on the head here.
He's a bit more advanced than most children his age-- he picks up on things quicker and is growing up a bit faster.
Your older children exposed him to some things that he is mimicking, which is what younger children do.
Stop the exposure, tell him plainly that this is inappropriate, and because he is so smart, you might want to explain why.
If it continues, you should seek help -- but if it does not, sounds like he is ok to me. Smarter kids are harder to handle.
You need to seek help immediatly. Not to scare you but these are the kind of kids who grow up to be phetophiles or worse.
There is deifinitely emotioal issues going on there and intellignce is irrevelent to that. Its an issue you have to deal with now because it will only get worse. Just becuase he says he hasn't been sexualy abused doesn't mean he hasn't. It's easy for abusive adults to brainwash kids into not telling their parents or a trusted adult about the abuse. If you get him psychological help, you will be able to get to the root of the problem. It is very likely that he was abused and isn't telling you. You have to get him help though, that is not normal behavior. Simply telling him that it is inapropriate isn't enough, yes you should as with anything that is but this is a much bigger issue. Please see it as such and get help for him.
Maybe boys will turn out differently only time will tell on that, i was the same at that age maybe a little older, again i had seen some things on t.v. i shouldn't have and this led to curiosity. but the other kids he is sharing with are equally as curious and are not being forced, are they? long and short is all i'm left with is a very healthy sex life.
Stop typing in all caps. It is really hard to read. Your son may be being sexually abused and hiding it from you because the person sexually abusing him may have bullied him into promising not to tell.