a boy of 8 does not know about pelvic thrusts or oral sex unless he's seen hard core pornography, or been abused himself. I agree, look into the home life of that boy. This doesn't pass the smell test.
You would be amazed how often this sort of thing happens, and at what early ages. Very few actually make the news. I deal with it regularly. Usually the parents are either druggies who have exposed the kids to pornography, either deliberately or inadvertently, or they're just completely oblivious to the effect that their own behavior, words, "entertainment", etc. has on their children. I have run across situations in which I believed that parents were actually promoting sexualized behavior in their children for profit. Darn hard to prove though.
Naw, he should be home-schooled! (/sarc)