Chicken is only used to entice turtles to eat that have been ill or not eating. Other than that white meat chicken (boiled, skinless) can be given occasionally as a treat. I'd skip it though if I were you and concentrate on the veggies/plant matter that he should be getting daily instead.
A mature RES should get pellets every other day, once-a-day, the amount that would fit inside their head if it were hollow. A variety of veggies should be offered daily, especially since a full grown RES is more herbivorous than younger RES. There's a list of what to feed and not to feed at the top of this section. Try feeding dark leafy greens such as Romaine, Green leaf/Red Leaf lettuces, dandelion leaves, etc. along with carrots, squash, pumpkin, zucchini, sweet potato, and red peppers. Even if he doesn't take to veggies right away keep offering them. Sometimes you have to apply tough love and withhold the foods they are used to for a few days until they begin eating what they should. The lack of the vitamins and nutrients from the veggies may show up eventually in his body, but it's never too late to make a change for the better.
