In that picture from ShowPeople, with his exposed tummy, someone said they thought it looked like he did have one, but she was the only one who thought she saw it.
That'd be really hot if he did, though. I suppose he could've just removed it during The Normal Heart, but he doesn't have one in Hair, either.
I've seen one too many men with nipple piercings. eek! It just looks too painful for me to find all that attractive, unless I just don't think about the pain.