My suggestion would be to keep his water as clean as possible and keep him well fed. They seem to be either eating or looking for something to eat at all times. If he's eating and is active, I would think that is a good sign.
Mine won't touch algae wafers but love their blanched vegetable slices. You may have to offer vegetables several times before they actually recognize it as food. Although I haven't tried this yet, I have read about people feeding "sheets" of algae that apparently are available at some online fish supply stores.
By the time they reach pet stores, they are in a weakened state because they're half-starved. Also, and I don't know if it's actually true or not, but I've read that when they're caught, cyanide is put into the water to slow them down so that they're easier to catch. So in addition to already starving, they may have also been poisoned.
Good luck with him and I'll be keeping my fingers crossed!