I have one in QT undergoing Hypo, He never ate prepared foods before, so this was a challange. I have wrapped Nori around a mag float, and he seems to be picking at it. I also place a spirulina sinking disk in the tank every day. I can't say he's eating either one of these for sure, but It makes me feel better. He is not as fat as he was, but still looks healthy. It's been 4 weeks now. He also picks at the glass some, so there must be something there for him to eat. Yours may do the same as the algae builds.
Do your best to get him to eat prepared foods, try everything. Try wrapping rocks, plastic tubes, and mag floats in nori. Place several in different spots in the tank. You just might fool him!