I would start with peanut butter or something he can "lick" out of the kong first. Building his confidence that he's succeded in getting the prize. Then try putting kibble pieces in (they easily fall out) but once again, its building his confidence. Slowly build up to things that take more time and problem solving and he'll figure it out.
Both of my dogs have Kongs and Bear will never give up! LOL He will carry it around all day until he figures it out. Mya, on the other hand, will try and try for a bit, then give up and forget about it. She could care less about the treat inside if she cant get to it.