What is the reason for repeated mouth ulcer in a child?

Hi doctor,

My son is 4 years and 9 months old. He is a healthy kid with normal weight (37.48 to 39.68 lbs). But, he often gets a mouth ulcer. It is quite often like once in two months mainly in his buccal mucosa and inner corner of the lip which touches the primary canine. This hurts him and so he avoids food. I used to apply Dentogel on his ulcers. My question is, what is the reason for this repeated mouth ulcer? Is he deficient in any vitamin? Initially, I thought it could be a traumatic ulcer. But, it gets repeated once every two months. Sometimes, he gets three or four ulcers together. I am really worried.

Please suggest what to do to avoid these ulcers.