My DS hasn't seen his dad since he was 3..
I would say the high point of question asking was when he started school, He became aware of everyone else's family, read about biff ,chip and kipper and there lovely family and saw dads on the playground.
I have always been honest and I don't want him growing up hero worshiping someone who isn't worth it. I tell him in an age appropriate but so it can be built on any time he asks.
I tell him I don't know why his dad doesn't see him and then explain I caannot imagine not seeing him for a day. He knows it is his dad's decision not to see him and unless he gets in touch it is nothing we can sort out. Now he is 9 in the past year he knows more and knows if his dad did get in touch he would have to go to court to make sure he is safe and the right thing for him. He knows about some of my ex's behaviours that why we left but again none of that at 5.
I personally wouldn't say you may see him one day as it sounds like it is a false hope.. I do say I don't know where daddy lives, he does have ways of getting in touch with us.
I would say though. I told my DS over and over again it was nothing he had done over and over again until the point he would just roll his eyes.
The thing that seemed to satisfy my DS was I had to got called in after topic in school "happy families " DS had said all sorts of things like I don't know if my daddy loves me in topic..So we all had a chat..His teacher said when you last saw him , you were a baby, and nothing you did nothing wrong.. He seemed satisfied with that.
I also had the bedtime questions. I realised I felt compelled to answer and sometimes put more importance on any comment about his dad. I now will say at bedtime it is a conversation for morning.