Like others have noted, there are great kids' books (and books for parents) out there to help you frame this in a truly age-appropriate way without going too far but still addressing his perfectly normal and natural questions. You also could talk to his school guidance counselor, who should be well aware of resources and ideas for discussing this.
One thing I'd add: Is there a way you can find out if the sex talk on the bus, in school, etc. is the normal curiosity stuff or something cruder or ruder? Are boys on the bus making crude comments about girls, or vice versa, etc.? (I hate to say it but if he rides a bus with older kids, crude talk, not just curiosity talk, could be quite possible; I know of parents who have had issues with the kind of stuff older kids talk about on bus rides around younger ones.). You might want to do a little investigation into what he's hearing. Again, see if the counselor is tuned in to this issue at your school. As for TV, if he's only 8, I'd direct him toward non-TV entertainment if he's watching shows that could have sexual content -- no kids' show should. I know some kids like reality shows etc. that can get racy, but maybe you can steer him elsewhere. Good luck!!