Abscess in older male rat?! Or tumor that has worked it's way out? Help!?

My very elderly, unfortunately obese rescue rat - vet says he's over 4 years old, has something weird on his side near his girth area. It's comparable to a human blackhead bc it's round, and had a hard, crusty m inside. The actual hole is the size of a dime and the crusty hard m is like a gum ball. Anyways I was cleaning it, applied some pressure and the m came out, leaving an open hole. Flushed w betadine solution and packed w tetracycline/polymixin b ointment. Any ideas ratty experts?