I would watch for anymore blisters especially since he's running a temperature. This sounds like it coul be coxsackie virus. Have you heard of it? My son had this very bad and it started with one or two blisters in his mouth and then within days they were all over the inside of his cheeks, on his tongue and lips also. It is very painful. My son would not eat or drink for 3 days. They don't do anything for it either because it's viral and has to run it's course. What finally helped my son (at the docs advice) was to mix together a bit of benadryl and maalox, then while wearing sterile gloves (no powder on them!) rub this on the blisters. Your son will NOT like this but it temperarily coats the blisters allowing a short time where your child can eat without so much pain. You have to have whatever food you're gonna give him ready before you apply this because it doesn't last long.