bleeding nipple?

My cat, Sam, a two year old male tabby, recently had to have his belly shaved for an ultrasound (all turned out fine with that).  But he has been picking at one of his (now exposed) nipples.  Today I noticed it was bleeding all around the nipple (it's as if he's been trying to rub it completely off!).  I have no clue what I could put on there safely and can't think of any way to bandage it.  Any thoughts on treatment or care?

thanks

herev!