I've actually done a little reading on this in the past. I think what's going on with him is, the part of his brain that keeps you stationary when you're dreaming is not working in him. I watched a program a while ago about this. Now mind you I don't condone this in any way, but the researchers had intentionally gone in and disabled the part of the brain of cats that paralize them in their dreams, and some of the cats shown were doing just wild things, it looked like some were fighting other cats, some were rolling around purring like crazy, some chased invisible things. So my quazi (but not really) educated guess is just that. The part of his brain that keeps him motionless while dreaming does not work.
Now that said, I sure hope he never actually hurts himself or others, and I seriously hope that his owners realize something is wrong, and get him to a vet, or do
something so he doesn't get hurt.