You can try a rinse that I have used for my son when he gets mouth ulcers. It is 50%maalox and 50% liquid benadryl. Have him swish it around in his mouth and then swallow it. It makes a tremendous difference for my son. It is all over the counter and you don't have to have a perscription and neither medication should bother any thing else he may be taking. Good luck.
Cindy