My Basset Hound ***** his nipples

My basset hound ***** his nipples until there raw and bleeding. I put A & D otinment on them. Once I get one cleared up he moves on the the next one. this is been going on for about a year now. Not sure why is doing this and what can I do to prevent him from doing this? He is the only dog we have. He is in a hug fanced in backyard. He doesn't like to be inside. He has a well maintained dog house full of new ceder shavings. When cold sets in we change out his bedding with new and fresh.

Help!
thanks,