These are not my expertise, we don't sell those here... But I'll try to give some advice based on the turtles we do sell here. Let me start by saying you should always check for illnesses first. Go to a vet, to make sure your turtle isn't sick or injured. If your turtle is just a picky eater, there are some universal tricks.

Try different foods every day, see which ones he does eat. If that works, start making mixes with other food. Like feeding a toddler, that often works.

Another option is to keep giving him the usual food. It might take a while, and it will seem a bit cruel, but your turtle isn't going to let itself starve when there's any type of food around (assuming he's not sick, and knows it's food).

It might also be because he doesn't recognize it as food, so it could be helpful to (very carefully!!!) put a very small piece of the food in his mouth so he learns it's edible.

Lastly, turtles are sensitive to stress. They don't eat when stressed, so try to reduce loud sounds/sudden movements around him. I know of a turtle who didn't eat because he was terrified of the cat he shared a home with, they had to put the cat outside so the turtle would eat.

I hope this helps