child with nasty mouth sores

my son about 6 months ago had an out break in his mouth after falling in the shower and busting his lip open. 3 different doctors gave me 3 different diagnoses. everyone refused to run tests to be sure what it was. it is now suddenly back.
he has a slight fever, really bad breath, red ***** blisters covering his mouth. he wont eat or drink. he is sleeping most of the day and when he is awake he cries that his mouth hurts.
the only solution we were given the last time was to make a mixture of benadryl and mylanta, and have him swish and swallow it.
is there something i can say to make his pediatrician run tests? i am very worried this could be something serious and no one is paying attention.

[Jerome Bogin, D.D.S.]

I can't diagnosis it without seeing the sores.But I certainly would take your child to either a pedodontist or an oral surgeon.