Sounds like he really needs LIVE food (blackworms, earthworms, mosquitos, amphipods), at least for awhile until he regains some weight. You may be able to train him onto frozen foods later, but he's not going for them now and will most likely die if you dont try live foods. Also, you may need to isolate him from the other fish for awhile so he can get his fill. He probably cant eat as fast as the other species in the tank. This is a common challenge with darters.
I agree, and have kept many variegates. Tougher than many. Really need live or be sure to get lots of blood worms down to them. Best bringing them home around an inch or so. Mysis seem to be an acquired taste.