Flakes are pretty much useless for big fish. Pellets are good; I like Tetra's cichlid pellets. Of course live feeders are good, but with live feeders there is always a risk of parasites. Don't feed him heavily on feeder goldfish. I think rosy reds are a better feeder; nightcrawlers, crayfish, shrimp, and various feeder insects are also good.
Frozen foods are good too. You'll spend a ton trying to feed him enough frozen foods from the LFS, at least once he's more than a few inches long. Try going to the grocery and asking for seafood that is about to expire; sometimes you can get it at a discount. The more intact it is the better; unpeeled raw shrimp with heads on are better than cooked, peeled, or headless shrimp, for example.
As mentioned, adult largemouth require a big tank- 200 gallons plus. Re-release is illegal and dangerous; if he gets too big to handle, please euthanize him or find another fishkeeper to adopt him.