Nikkicom, check out this section on pellets.
http://www.redearslider.com/pellets.html
This gives you the best guidelines on the quantity to feed your turtle. There's a paragraph in the middle of the page that says exactly what to do. Since your guy is still so small, and he's under a year old, you'd feed him like a hatchling - ie, a "headfull" of pellets a day plus veggies. Occasionally a treat like freeze dried crickets can be substituted for the pellets.
How does this compare to what you've been doing?