how do I explain to my 12 year old boy that's he's he's going threw puberty?

Jamie • Well ....
He is asking me why his "pollito" won't stop staying up and that sometimes he is waking up with a little pee in his underwear , I want to explain to him the right way to help him understand , his dad is out at sea and can't talk to him el he's back next week 
Any moms go threw this yet and have some comments please advice