Feed earthworms, small pieces of fish filet, shrimp, Hikari Sinking Carnivore (you may have to cut this in half), bloodworms, etc. Yeah, he will eat that, but these are the foods that I had success with when my RTC was just a tiny guy. Also, be sure to move him out of tanks with other fish that even remotely look like he can eat them way before he gets to a size where it's a risk. There are too many stories of RTCs eating Aros almost the same size as them.
Disclaimer: This fish will grow to at least four feet and require a minimum of 1,000 gallons to survive. This will happen quickly. Typically, growth to two feet takes place in the first year.