They'll poop when they're good and ready. The easiest way I have discovered to prod them into going is to clean their home top to bottom. Once you're nice and happy with it you're sure to get a big wet poo smeared up against the front. You may even hear when he goes, which is quite something. My little normal has woken me up from a dead sleep with the sound of him going both when his tank was still in the bedroom and after I moved him into the loft upstairs.