Thanks again everyone! As Ginger had suggested it isn't a major issue. While it isn't a normal boo-boo, it isn't life threatening. I was very worried considering his age.
The lesion was closer to his bum rather than on his belly. As soon as the vet saw it she said "I know what is going on here." As yucky as this is she said that he had an anal sac rupture. Said it was probably inflamed or blocked from infection and that he had done a good job of cleaning it up himself. He is now on antibiotics and we go back for a visit at the end of the month. The vet said this is uncommon for cats but fairly common in small dogs.
While at the vet today I asked about the average lifespan for indoor male cats...the vet just said, "he's old." He doesn't act it though and I think that is all that matters!