i mean you could put a juvenile oscar in a 5g, and it won't get as big as one in a 100g tank because you've stunted it's growth, but thats inhumane. while i do think that flatheads are a pretty cool species and it would be awesome to have them in a tank, you're either going to need a massive outdoor pond to put him in, once he outgrows your 220g, or basically have an indoor tank of several thousand gallons, taking up most of a house.
you also don't want to keep him in your tank, then re-release him back into the wild. Once in the tank, it's possible that he'll be exposed to bacteria or parasites or other diseases not encountered in the wild if your area. If he is re-released, he could introduce a disease or parasite to the area that the other fish cannot survive and it could lead to devistation. Ontop of that, once the fish is used to being fed, and not hunting for food, he might not survive back in the wild.
This is just my opinion, You can do what you want with fish and your tanks and either way i do wish you the best, i just think you should re-think the flathead and maybe go with a species that doesn't get quite so big.