J.T.
S., it could be a fungus other than athletes foot. My eldest son came home from camp with a fungus on his hands. Luckily, I had some prescription anti-fungal cream from his younger brother and it worked fast. I would first try some anti-fungal cream instead of the power- so that it stays on his skin longer. Has he been swimming? It could be from the pool. If you try the cream and it doesn't seem to be working, then I would take him in.
Another thing could be a weird allergic reaction. Then I would try some benadryl or hydrocortisone cream. Maybe you could try one treatment on one foot and another treatment on the other and see which one is working?
Good Luck!