If he hasn't started dehydrating himself at all, then he should be able to do 5 lbs. easy via standard dehydration. Tiny wrestlers can cut that much and MMA fighters routinely cut a lot more than that in 24 hours.
If he's still 5 lbs. over this afternoon, he should stop eating and drinking, take a hot bath, get a good night's sleep, wake up in the morning, and take a shit.