If you've tried the suggestions above and are still not getting results, here are some more suggestions on getting him started:
http://www.redearslider.com/entice_eating.html
Avoid stress, which means don't pick him up unless you have to. He won't eat if he's too nervous. He's probably still adjusting to his new home. It may help to cover the sides of the tank with cardboard to give him more privacy.
Sometimes turtles are eating when nobody is around because they don't like being looked at when they are eating. If you are feeding him pellets, count the pellets you put in the water and then leave the room, and see if any are missing when you come back.
Once he gets used to you and his new home, and the temperatures are right, he'll calm down and will happily eat in front of you.