Being dry at night is rarely something a child can do much about; it can click any time from 18 months, and normally by 7 years old. But there are also many children for whom it takes a lot longer, through no fault of their own.
Night dryness has no relation to day time dryness.
You can't train for night time dryness.
Well done of the boy for still feeling able to have a sleep over and just get on with it.
Definitely don't say a word to him about it.
Dd is 6y and has only been consistently dry at night for past few months, still has had the odd accident but now more and more rare - thankfully for her as she hates it. She was dry in the day from 24 months, entirely self led within 2-3 days. Definitely not for wanting to e dry at night, that's for sure. Fortunately her two best friends with whom she has had sleepovers with have never said a word, although she hasn't had pul ups since turning 4 but she dd have a reusable bed mat in her sleeping bag.