Divorced Dad Diary: Do You Have Cure For Canker Sores?

I've got a kiddo who is dealing with something very unpleasant; Canker Sores.  He keeps getting them and I'm hoping you might have found a solution that you're willing to share.

I remember getting them as a kid, and the solution from my parents was alum - it worked, but man was it unpleasant.

If you deal with cankers, have you figured out a cause and more importantly, a dependable solution to get rid of them?

Your advice is appreciated!