Poor kid. It sounds like he was trying to look up wrestling moves, which he is genuinely interested in, and unexpectedly found sites that were sexual in nature. I would say that you should buy him a good book about wrestling moves and positions to practice so that he doesn't have to look them up on the internet anymore. And I think it's great that you are willing to practice wrestling with him; there is nothing wrong with it at all, it is a sport like any other sport. Have fun!
Also, maybe try to get him into a mixed martial arts class or something; I know they do a lot of wrestling and he might have fun with it.