Fantastic GT, and a huge hump for only a 6" specimen !
The hump is actually completely made of fat, so I doubt a high protein diet has anything to do with it.
There is a huge amount of folklore around in the flowerhorn world about getting and maintaining huge nuchal humps. Most of it is BS. In my experience, to get the best hump, the fish needs to feel secure and dominant in it's environment. I had one FH whose hump shrunk by half in a couple of days once, when I introduced another fish into the same tank (the other side of a divider). It went from being the undisputed dominant fish, to having competition, thus the hump shrinkage.
I am also aware though that a lot of FH keepers are convinced of the opposite, that introducing a fish in competition (especially for short periods a day .. usually done with a mirror) will actually increase hump size. I suspect that 24hrs a day of competition is usually too stressful and the hump will shrink though.
Also, needs to be well fed, obviously. Although IMO, there are no special types of food that are needed, just a good mixed diet.