Hard to predict!
If one starts out normal, by the end of 1 week of no pooping, bad things should start to happen.
Maybe severe abdominal pains. Maybe liquid poop will leak out the anus. Then liquid poop may come up as vomiting
(looks very scary). If only leaking liquid poop, and not vomiting it, he could go on for a long time if he is able to eat, maybe months. If he can't eat, maybe by week 4 he will start dying.