I used to suffer from these regularly.
The only way I found to 'nip them in the bud' was to lightly rub off the 'scab' (the white part) and then clean the sore out with Glyoxide. It was pretty painful, and again, I had to use it right when the sore started developing, and it seemed to prevent them from turning into massive craters. If they'd already become large before I ate/drank something that irritated them, I'd have to wait it out, but holding a wet tea (like black tea, not herbal) bag in my mouth as a poltice seemed to slow them from getting worse and seemed to help dull the pain.
I started keeping a record of when I started developing them, and noticed a hormonal pattern. Since I started boosting B-complex and Zinc vites the week before I typically developed them, I've not had a problem--even when I bite my mouth accidentally.
If he gets them regularly, it might be worth jotting down a simple food log for a month, just to see if there's a pattern after eating citrusy/spicy/sugary things, or if he's highly stressed, ect.