Try feeding some raw hamburger and see if he takes to that! As I said they are carnivorous and cat food may be healthy (well I've never heard of feeding them that before, personally...) the smell of meat might entice him to eat. Also try some bloodworms or ghost shrimp.

I made the mistake of keeping my RES in a 10 gallon when I first got them... one got pretty sick. But I had more than one of course in it. I suggest upgrading to a 20 gallon breeder/long to see if that would help at all??