J.C.
Like the others said, kids pick these things up no matter how careful you are. Same with spelling them, for some reason I've seen kids who couldn't spell anything else but could spell the sh & b words..? I had a neighbor chew me up and down one time because a child at my house (not mine & they knew that) has Cerebral Palsy & was singing a foul word while jumping on the trampoline, just nonchalantly & they were highly offended. Too bad, wasn't my kid & I was watching them through the window so I couldn't hear what he was singing...Sorry. I jsut always reiterated teh fact this wasn't a nice thing to say & would try to replace a nice thing with what they had said so they would learn the difference. I don't know of any books off the top of my head that deal with that directly, but there's a set called the Help Me Be Good Books & they may have one in the set. They address several different child behaviors & the kids like them. Good luck & he will get over saying it, eventually :)