Do you only put that many crickets in, or more and that is all he eats? Different beardies grow at different rates.
With your uvb, the contact bulbs even the repti-suns have been known to cause problems. With out the proper uvb, it can cause stunted growth. The only safe and highly recommended bulb in the US is the repti-sun 10.0 linear tube. With a 40 Gallon breeder, a 24" would be ideal.