I've found out that turtles can go for more than 3 weeks without eating. It can take several weeks for a new turtle to become used to his new environment. The stress caused by change can make them not eat. Since he has eaten a little bit, he shouldn't starve. I've also read that giving him dry cat food is okay once in a while, as long as the food has been rehydrated in water mixed with calcium powder (just enough water to rehydrate the food).
You should also establish a routine. At a specific time each day, put a few pellets in the water and walk away. When you come back, if the food is still there, take it out of the tank. He should get used to this routine and start eating.
Just remember that if the tank is not warm enough, turtles won't eat much.
