Bump under mouth?

Any thoughts on what it could be?

Looks like a rodent ulcer, or eosinophilic ulcer.  It could also be chin acne, but that usually looks a bit different.  If you're not already feeding him from stainless steel, glass, or ceramic bowls, I'd switch to those immediately.  You can also use paper plates.  Get him to the vet for treatment, too.