If he hasn't been pooping you can try a warm honey bath. Put a few drops of honey in warm water no deeper than the frog's chin, soak for 15-30 minutes, then give a rinse bath in plain warm water to remove honey residue from his skin.

Has he had a fecal test? The other likely culprit here is parasites, which you would need to bring a fecal sample to a vet to have checked. This sounds exactly like how one of my whites was and he turned out to have parasites.

Also, I might lower his humidity a little if I were you. Whites are pretty versatile for humidity ranges, but generally 40-60% is good with 50% ideal.