How much smaller is he than his cagemate and how long has he been lethargic? What is the temperature on the hot and cool side of the enclosure and what is the humidity?
Not eating doesn't necessarily call for concern, unless your leaving crickets all night and the frog still isn't eating after some time. Some frogs have a real hard time with light, and absolutely will not hunt untill a few hours after dark. That could be the problem...