You should leave a dish of crested gecko diet in his enclosure at all times. A lot of times they are eating it, you just can't tell because they aren't eating a lot (their stomach's are pretty small).

Crickets can be fed one or two times per week. They should be no longer than the space in between the gecko's eyes. Make sure they are gut loaded and dusted with a good calcium/multivitamin powder. They can eat crickets pretty much from the time they are born as long as they are appropriately sized, so I'm sure he can handle them.