How to tell my son about the baby on the way

I’m about 7 weeks pregnant. The pregnancy was an accident from a one night stand, and the father wants nothing to do with it. I told my almost 5 year old son about the baby and he’s very excited, but one of his first questions was who the daddy is. I avoided the question and sidetracked him for the time being, but I know he will ask again. He’s a very smart kid that needs to have answers for everything. I have no idea how to answer this question in a way that he’s going to understand. Has anyone gone through anything like this, or have any suggestions?