So he's what, 3 inches long or so? I would say 10-20 vitamin dusted crickets once a week.

Humidity is not really a concern if you are keeping them semi-aquatic like you should be doing. Mine are housed in a half land/half water setup that keeps the soil on the other side moist (I design them in such a way as the pump is on the land side, and uses the gravel making up that side as a filter, over which soil is placed, kept moist by capillary action) and well planted.

If you are keeping the frog terrestrial... dont.