at that size they love brine shrimp, also hell take some cut up squid. they get about 15 inches and when they get a little bigger theyre pretty hardy. good luck!
Move him into a tank with a sand substrate. Not to thick just enough for him to cover himself. Light to moderate filtration.
Subdued lighting.
Feed frozen or live blood worms.
Consider getting a small herd of platties or mollies or guppies for that matter.
These live bearers fry will also serve to feed it.
I have this kind of set-up in a 33gal long. Some minimal planting and java-moss. The Flounder grew from 1.25 inches to 4.75 inches in the past 4 months.
Respectfully, FireMedic