One of my burms is no pooing!! Help

My male albino burm will not deficate unless i soak him in luke warm water. He just started this 3 months ago. I feed him medium f/t rats every 2 weeks just l feed the female. I first noticed it about 6 weeks ago and just soaked him and he pooed. I thought it was a one time thing. But i went in tonight to do my check and had noticed that he hadnt pooed and his area down by his vent was buldging just like last time. So i soaked him just like last time and he pooed just fine. Why is he doing this? Any help would be great.