If it's showing normal behavior (aggression, aquascaping his new home, etc.) then it's probably not sick. Try giving him something other than flakes, which probably aren't the most appetizing food for a cichlid with a big appetite. I always feed smaller newcomers frozen Hikari bloodworms. I've yet to find a fish that isn't tempted by those. Once you have them eating, then ween them onto things like cichlid pellets, etc.
If it hides every time you come around, maybe it's just still adjusting to its new home and it needs to settle in. Cichlids can be shy until they get to know you.