This is just my advice as a no-one on the internet:

You and your father both seem really mature about this, which I respect a lot. It seems like a good time for the three of you to sit down and talk about it with your son. He’ll probably have a lot of questions and get emotional, but just give him patience and help him understand things. I don’t really see much of a reason to buy a book to do this, but I don’t have much experience with small children either