Why isn't my bearded dragon pooping?

I got him around christmas and he is a two year old male. At first I thought the sand was impacting him so I switched to bird seed that was recommended to me by a bearded dragon expert. The problem has gotten better since then, but he still only goes every 4 or so days. I feed him kale and all the crickets he'll eat every say and. All the temperatures are perfect. So what could the problem be?