S.B.
Have you talked to him about it? About personal space and not touching each other's private parts? My 6 year old knows about private parts and how we don't touch anything that a bathing suit covers. (Unless we're at the doctor or someone's in pain. She gets vaginitis occasionally and so the doc needs to see her private bits or we have to put a cream on it or something. But she knows that that's only ok if mommy or daddy is there.)
He's definitely at the age where he should be able to understand if you tell him no and explain to him why.