Hiding Pull Ups? Help Night Training!

My boyfriend’s 5 year old son has been hiding his peed in pull ups under his bed. Obviously, this is not sanitary, but sometimes we won’t discover them for a few days. We aren’t sure why he has been doing this because we use positive reinforcement to let him know that we are excited when he hasn’t peed but also if he does we are not mad at him.

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We haven’t been able to night train him. He is at his moms 3 nights of the week and our house 4 nights of the week. We feel defeated because we can’t control what goes on at his moms house but whenever he goes back over there it seems like we need to start over with the training. She just won’t comply. We haven’t been successful with waking him up to pee and stopping drinking right before bed because he has started to sneak water lol. HELP! Anyone with 5 year olds still night training??