Too much Vitamin C can cause canker sores. Try having your son eat yogurt, any flavor, to help balance his ph system. Lysine may also help. But yogurt is a cheap, easy, nutritious way to keep from getting them. Also, whenever I start to feel that tingling sensation and know a canker sore is coming on, I take 4 aspirin every 2 hours, and usually 3 doses will stop the canker completely or halt it from growing. If I get any at all, it's just one tiny little fast-healing spot with the aspirin. If your son has stomach problems (and depending on his age) the 4 aspirin may not be appropriate, but they've sure helped me.