Yes, sleep sex is a very real thing. People have been known to do very strange things while sleeping. I had a cousin who would talk in his sleep, but it sounded like Russian, but if you could interrupt his very vocal conversations long enough, you could actually talk to him and get him to do simple routine things. When we were kids, we would get him to do all sorts of crazy things, then not tell him about it, and try not to laugh when he would try to figure out why he was sleeping on the floor, or something when he woke up the next morning.
I was a sleep walker and was known to do some strange things like talk on the telephone and try to turn on the the t.v. I actually left my house one time in the middle of the winter and I was barefoot. I woke up outside very cold and very confused. I am now finding that my son is starting to sleep talk and walk as well.
There are certain sleep medications that actually make people more prone to these kinds of behaviors, but I'm not actually sure if there is really anything he can do to make him stop, but it certainly doesn't hurt to have him talk to his doctor about it if he hasn't already.
I do know that the body naturally produces a paralysis chemical to keep it from moving around during sleep, but in people who sleep walk, etc., this process is impaired somehow.