First thing I'd do is announce, "I want to remind everyone to remove all jewelry, including body jewelry like earrings, nipple rings, lip piercings..."  I'd even do the last bit with a chuckle to take the pressure off.

If that doesn't work, I'd cut to the chase and go tell the actor that I could see his nipple ring through his costume and it's distracting.

I think a wire cutter should get his attention.  But seriously be up front and tell him that they need to be remove for performances and dress rehearsals.