The answer is lots of Vitamin C (Ascorbic Acid).
Go to a pharmacy and ask for "Redoxon Double Action Vitamin C Effervescent Tablets". They contain 1000mg (1 gram) of ascorbic acid plus a small amount of zinc. Some years ago, Redoxon was available without zinc, but these no longer seem available.
If your son is aged between six and twelve, the dose is half a tablet dissolved in a tumbler of water per day. If he's over twelve, he can have a whole tablet dissolved in a tumbler of water per day.
Start him off on Redoxon as soon as the tingling of a mouth ulcer starts and it should help prevent the ulcer from fully forming. Healing of the lesion will also occur more rapidly.
If he has an ulcer right now, start him off on the treatment as soon as possible and treat until the ulcer has fully healed over. This should be less than a week.
Don't give him more than the recommended dose I've suggested daily and once the ulcer has fully healed, stop the treatment.
Once you're happy the treatment/prophylaxis works, you can buy 50mg Vitamin C tablets over the counter at pharmacies. Giving him one a day will probably keep them at bay.