Besides temp and humidity levels, what water source are you using and what supplements are you supplying when feeding?

Discoloration and blotches or spotting can be many things and its hard to diagnose without more info such as pics of enclosure, how well he is eating, and if you are finding normal droppings from the frog. I have seen skin issues similar from handling after handling something toxic to the frog and from actual bacterial issues.

If I were you I would send a photo of the frog to Dr.Frye.dvm@sbcglobal.net and explain the condition to him. He can also be reached at (734)439-2273. He can send you medication to treat the frog if needed.