You don't need to wait until he poops. He may not poop for a few weeks. A few of mine do that. Eat weekly, poop every 2 or 3 weeks.
I'd just feed him once a week. He'll be fine and he'll poop when it's time.
Also, make sure it's big enough. The rat should be about as big around as the snake is wide at it's widest point. Just out of curiosity, do you know how much he weighs now?