You don't need to feed him at all. I was just surprise that the was still small after 10 years in the tank. In no way, I am imply that he is not healthy or you are not taking good care of him. He is healthy and beautiful in your pictures.
The more you feed the faster he grow. I have a Haddoni carpet in a 24 g tank. I don't plan to feed him much so he won't grow to more than 6-8 inches or so. My carpet is tiny only 4 inches in diameter, so I am feeding him now. Once he reach the size I want, no food for him other than the fish food that he catches, or if he decide to eat his clowns.
I also have a tiny H. mangifica in the same tank at this time. He is only 2.5 inches in diameter. I don't feed him much either because I don't want him to grow until I set up a tank for him.
The reason I bought him is that he is a very beautiful find from LFS here. His column is 1 inches purple, Oral plate is only 2.5 inches fully inflated and is green. His tentacles are green lower third, fade to tan, then white top third with a blue/purple "eye spot" at the very tip. I normally don't buy animals, then set up a tank for them. This is a special case because he is so small, and is exceptionally colored. I think I can safely keep him in my 24 g tank until setting up another one for him.