M.H.
Hi H.,
L-Lysine will prevent them but if he gets them a lot then his immune system is not working at it's best. That can cause other problems as well and you should see this as a red flag. He needs to be on an absorbable multivitamin/mineral complex daily. The vitamin will help a lot and I would recommend keeping L-Lysine in the cupboard in case a canker sore starts to appear. If he gets them often then he probably can feel one coming on before it gets full blown. I would also recommend an L-Lysine supplement that has the B vitamin in it. That will help it absorb better and he won't have to take as much. L-Lysine has few side effects, none if only taken when needed.
Dodge places like Walmart or GNC when shopping for it. Get it from a pharmaceutical health store.
Regards,
M.