21/2 year old ulcers in his mouth , fevers, dr said viral infection but i disagree, what else could be causing the ulcers?

He is likely correct: Those symptoms are consistent with a viral infection that is not uncommon in small children and is usually caused by a virus in the herpes family. ( not an std ). It can give high fevers a mouth full of ulcers, lymph nodes and generally feeling crummy. Biggest concern is maintaining hydration and may need to use a special rinse to provide pain relief to allow drinking. Can last days to a week.