J.W.
My son had the same problem. He has eczema issues so I thought that is what it was..plus he wears crocks all the time and I thought it could be from sweat and the rubber. Well..nothing really cleared it up..so I took him to the doc and it was athletes foot. They gave us some meds. and it cleared up. If you have used stuff for athletes foot and it didn't work...then I would think it either isn't strong enough for his problem...or he has something else going on. I would just take him into the doc and get to the bottom of it so it can be taken care of.
Good Luck!!!
J.