What does it mean when a guy has a girlfriend but always wrestles around with you?

It means that he likes you as well, and possibly in a romantic way.

It's up to you to figure out whether he's serious; and if so, whether it's worth it to your friendship with his current girlfriend to take things any further.

If it's not worth their breaking up, or if you don't have romantic feelings in return, then nicely let him know that Wrestling around with you while he's in a relationship isn't right.