Kick his ass ant take his shirt off.
My boyfriend was nervous about taking his shirt off for a while because he has a pretty bad scar from a surgery he had on his heart when he was a baby... He got over it though... If this guy isn't even mature enough to tell you why he likes to keep it on, he has problems.
What if he took it off and he had bigger nipples than you do?
It could be a number of things.
He could be really insecure about his chest for some reason.
He could have serious scars as was suggested.
He could have offensive tattoos; you know, such as a bare breasted girl whom you might recognize.
He could have male tits from hormone imbalances or something.
Get him drunk one night, do him, and then wait until he's asleep one night and check out his chest.
Then write to let us all know what the secret is!