Here's some info on feeding pellets:
http://www.redearslider.com/pellets.html
In a nutshell, imagine that your turtle's head (just the head, not the neck) is hollow, like a thimble. Then, if he's less than 4" long you can feed him as many pellets as would fit in his hollow head every day. If he's more than 4", feed him as many pellets as would fit in his hollow head every other day.
One of the keys to a good turtle diet is variety. A turtle should have green leafy things every day (whether he eats them or not, they should be offered) so what you're doing is good. In addition, you can sometimes offer him some blueberries or an apple peel or a carrot peel. You might put water plants like anacharis in the tank for the turtle to munch on from time to time. Every so often he can have a shrimp or some dried gut-loaded crickets or an earthworm. Mix it up and make it interesting for him.