Welcome to TheSquirrelBoard
Here is a link to baby squirrel care.
https://www.henryspets.com/1-baby-squirrel-care-guide/
It’s difficult to tell what the problem is with the wet belly. There might be nothing wrong at all. I have had a few babies that just seem to wet themselves often. Some are perfectly dry and others are soaked. Change the bedding often and keep the baby clean. Another way to help keep the bedding clean is to stimulate the baby to pee after feeding. You can use a tissue or paper towel. It will help to keep the bin clean. Mama squirrels stimulate the babies to pee and poop. It’s usually not necessary for 5 week old babies but if it keeps them dry, it can be helpful.