Undigested crickets can mean a few things, most of which has already been mentioned, so I won't repeat the same thing. Make sure his basking spot is reaching appropriate temperatures (for his age, between 105-115F is good). Also, are you sure it's poop? I ask this as it is also possible he regurgetated. They sometimes do this because of parasites, improper basking temps (ie too low) or eating too much at once. If you are positive it's poo (ie brown and smells like poo, instead of mushy and more fluid than brown stuff), are the crickets actually whole crickets or just the undigestable chiten (exoskeleton)? If just chitin, you may be feeding crickets that are too large or too tough, in the case of using wild caught. Most feeder crickets are not the same as wild crickets (feeders are house crickets I think, wild ones can be black crickets and other species, but I am a bit fuzzy on what the actual specices are).
One thing you can try is add silkworms to the diet instead of crickets. These are soft bodied worms (catapillers) that are much easier to digest, have plenty of water, calcium, protien and other nutrients, are a bit higher in fact than crickets though for growing bearded dragons, this isn't as much a problem.
-----
PHLdyPayne