Cresties will eat something known as Crested Gecko Diet, and you can get it online or at any pet store. You mix it with water in the proportion it says on the little bottle and voila! Mine love CGD, but if yours has never had any then it might need a little while to get used to it. Try putting a small bottle cap or something with a tiny amount of CGD along with a couple crickets. The maybe try reducing the amount of crickets he gets and see if the CGD starts disapearing from the top. And when you see he's eating it then you can give him more and more of it.
At least you're asking questions instead of taking care of him incorrectly

Feel free to ask any more questions you have.