The vet said "because he found out that he has nipples"...

It sounds a bit dodgy to me...all cats wash themselves and would know that they have nipples! Sorry to sound a bit surprised but it just seems like an unusual thing for a vet to say. Perhaps it was their way of telling you they didn't know?

It sounds like a behavioural problem. It may be due to an environmental change - you know, change in home, people around him, other pets, or diet?

Any ideas?