My boy does this a lot. But we started cutting down the liquid before bed, but also make him Go for the toilet tight before sleep, and now he very rarely is wet. We also, before this did double nappies for a while, and that did work
Is he dry in day time? Out of nappies? If so, make sure he goes for a wee right before bed and also first thing in the morning, and some people find lifting them to the toilet when mum and dad go to bed, helpful too.
We had this, I sized up a nappy and it stopped the leaks !
Is he dry in the day? Is he going or holding it? Sounds like he’s doing a lot at night, rather than the nappy. Have you tried cutting out a drink at night and making sure he’s had a wee as part of his night routine? Also if he’s waking wet at the same time, try waking him before that time to go wee then go back to bed after?