Best of luck with finding him a new home and enjoy him while you have him; they're a gorgeous fish. Plenty of water changes, with warm dechlorinated water and daily testing and you should be fine.
Did you say somewhere else he wasn't eating? Do keep on trying him on fresh fruit and veg; if he's never been fed it, he might not recognise it as food yet. You could try broccoli stalks, split and lightly cooked, blanched lettuce (a nice green one like Cos, thought, not iceberg; there's no goodness in that!) or slices of apple held down with a teaspoon.
Of course, I keep calling it 'he', but it could be a 'she'; there's no way of sexing these fish, to the best of my knowledge.