But he is past his primary growht spurt. If in the wild he would be grazing on vegetation at this age, why not give it to him in captivity?
What brand(s) of pellets? I would use mainly Zoo Med and Hagen Max.
I found that the best diet is a mix of a variety of high quality turtle pellets and high quality fish foods. I would not feed freeze-dried, frozen, and definitely not live. Live food is not nutritionally good for them, can increase aggression, and can introduce a variety of parasites and diseases.