As has been posted it would be good to find out what the temps in the tank are - too cold and they wont wanna eat. If you have just got him its quite usual for them to go off their food until they settle. It might be worth asking the vendor as well what they were feeding him on as they can become fussy. I wouldnt worry if otherwise he looks and appears healthy but do keep an eye on him. Other things to try are earthworms, waxworms (sparingly tho as they can get hooked!) and maybe small hoppers as well as the crickets. Dont forget to dust them regularly as they grow so fast.