Try cichlid colour pellets, Hikari has a couple of types, you want the sinkers I would think. If the pellets are a bit small and don't get him interested you could use them to "gut load" feeder fish or make a little cut in a fish fillit and put some in there. In the wild thay also eat fruit, especially dates, give them a go for variety. Shrimp should be good for colour too, but make sure you are leaving the shells on, thats where the colour goodies are at. Try him on parboiled (and cooled) pumpkin and carrot, loads of colour enhancers in both. Any fish or seafood should be good, but not too many goldfish as the have a bad enzyme, I think it is thyaminase. Try crayfish (crawdads in the US). I am not a fan of feeding warm blooded protein (chicken, mice, beef heart, etc.) to any fish, but I am not saying it does not work for some people. I just think that fish have a hard time digesting warm blooded protein and fat, so it just passes through them and pollutes their tank water. IMHO. Any way, good luck with the RTC, and how about a picture of this bad boy?
Cheers,
Ken.