its called a blinded penis. Very common and they tell me that boys grow out of it- otherwise they can do lipo on it.
speak to GP about it.
they can check his size etc against charts to make sure that it is in the 'normal ' range, otherwise he can have testerone shots which make it grow, or he can have the chordal muscle cut which brings it out of the body more
HTH and good luck
as long as it works and it doesn't give him any trouble - should be ok
vaseline works well to stop it rubbing as wel