Bearded Dragon. Lazy or ill?

I would make sure that he has a heat lamp at all times. If bearded dragons do not have that sometimes they will not eat as much, and will slow down just from being cold. I would try dusting the crickets with calcium when you buy them (you can request this at most pet stores) and stop feeding him all the locusts, that's not healthy to have as a solitary diet. Eventually he will give up and eat the dang crickets, but he might like it better with the calcium, and that will give him more energy and strength. Him being out of the tank and not getting enough heat might be contributing to the problem. Good Luck.