Here's the section on enticing eating:
http://www.redearslider.com/entice_eating.html
One thing comes to mind. You might try crushing some mealworms and pellets together (don't do this after you've eaten!) to make a paste and give this to your turtle. If he goes for it, you could gradually reduce the percentage of mealworms in the paste until at the end it's all pellets.
Of course, if it's the fact that the mealworms are moving that makes your turtle go for them, then this won't work.
Another thing - if tuna juice doesn't interest your turtle, maybe it's because he's not interested in tuna. Maybe you could try mixing pellets in a little bit of something else smelly - like cat food - and see if he'll go for it. My turtle loves shrimp above all else. If I mixed pellets with shrimp I'm sure he'd eat them - but then he eats pellets anyway.