Odd that you should post this today.  I just returned from my son’s pediatrician and was asking the same question.  He is only 13 months however like your son his diaper(nappy) is completely soaked in the morning along with his undershirt and pjs.  I asked his doctor, he examined his labs we ran two weeks prior and didn’t see anything out of the ordinary.  He said it might just be him.  I thought maybe running the humidifier my be contributing as we live in Colorado and it is extremely dry but even when it is not on he is still really wet.  I don’t remember it always being this way only the last month or so and I can’t think of anything that has changed much.   We did add whole milk to his diet rather than straight formula.  He is also eating more table food.  My only thought is that he is getting more salt from the table food than he was previously getting but I can’t see why that would make him urinate more?!

I will also be waiting for others to reply and see if we can make sense of this...