Even so, he is constantly playing with himself! Is there something wrong with him? Why does he need to play with himself so much? Does he have a disorder?
The "disorder" you boyfriend suffers from is called "being a male." This is simply how many males behave, whether in relationships or not. The reason why you haven't heard about it before is because it's a highly taboo subject. (We're trying to change that with JackinWorld!) If it helps, consider that at least he's being honest with you – most guys who do this slink around behind their girlfriends' back, masturbating in the bathroom or when the girl is away. I'd say consider yourself lucky – I'm sure he'd like you just to let him enjoy himself.