My first thought was scale rot but if its the only spot then probably not. Could be a tooth abscess but without looking inside it would be hard to say also.
Most likely it is a wound that is healing over. You can put a small dab of a triple antibiotic that has no pain reliever in it.
You can check the teeth and gums by holding his head with your thumb under his chin and moving it (your thumb) sideways to lower/open his lip like this V