Feed a high-quality, varied diet including a good brand cichlid pellet (NLS and Hikari are recommended) as his staple diet, with frozen bloodworms, frozen krill, shrimp, etc. as treats a couple times a week. Make sure you have a big enough tank (I would say 75G minimum for the flowerhorn alone), and keep the water quality pristine with excellent filtration and regular water changes and gravel vacs.
The development of the hump (kok) will ahve more to do with genetics than anything else, though good diet and clean water help. There are methods people use to enlarge the kok but I don't know much about them.