• See doctor if more than 4 sores in the mouth or if sores last longer than 2 weeks or if ocur more than 2-3times/year
•  Otherwise, ensure your son has 15 mcg of zinc daily or, if a higher dose, then take 2 mg of copper for every 30mg of zinc (taking zinc for longer than a month may interfere with copper absorption.  Young men are v. often deficient in zinc which is required for a huge no. of actions esp. for the immune system.
• 500mgVitamin C and 250mg flavenoids 3 times/ day
• One tablet vitamin B complex (with at least 50mcg B12 and Biotin, 400mcg Folic Acid & 50mg of all other B vits,.
• Use Echinacea 2=3 times a day as preventative or at first sign of a sore. High dose and then reduce  over 3 weeks of each month.
• Chew 1-2 DGliquorice tabs (380mg) 3-4 /day
• Pop goldenseal tincture onto sore during day but wait about an hour before eating. 
• Avoid coffee, spicy foods, chewing gum.

Good luck.