My friend has a 5 year old who they escaped from to a refuge when she was 4 months old. She's had no contact since. When her little one asks about dad, my friend just says she isn't sure where he is and she has "lost him, oops". It's enough to placate the child just now, but not sure what the tactic is for later.
I have another friend who, weirdly, also escaped from her ex when their child was a baby and has no contact. She pretends that the new husband is the 5 year old's father.
I would be honest, because if you're not it's going to come back and bite you. Obviously, you can give an age-appropriate version now and elaborate gradually as the years go by. I would probably start by saying 'Your dad is a man called Dave', and then see if he wants to know more. Having a 5-year-old boy myself I can imagine DS1 saying 'Oh, okay, and then running off to play. There must be books around that help parents to have these conversations - after all an absent parent, for whatever reason, in not exactly uncommon.
I have a similar situation to you - my youngest dd has never met her father. In her case it's hard because her older two sisters still see their (different) father a lot and she can't understand why she doesn't have a dad to go to.
I think you do have to be honest. In my case I'll say that her father lives a long way away if she asks (which is true). There is no easy answer to it though. I often wonder if she will (and hope that she won't) grow up with an idealised desire to want to meet him. He is not a nice person and she will be much better off with him out of her life. But it's hard for a child to understand that some people are up to the job of being a parent and others just aren't.
Im in the same situation with my dd (7yo), it is difficult and dd does get upset about not having a daddy. I have always tried to be honest and matter of fact, 'your daddy didn't want to be a daddy but i wanted you so much' is what I mostly say.
I also acknowledge it is difficult for her and try and give her as much info on him as I can which tbh is quite limited, she has seen a photo of him as well. I think if you have a new dp it probably won't be too much of an issue as ime a father figure is normally enough for kids to not feel they are missing out too much!!
I think telling kids someone is there real daddy when they are not is asking for trouble tbh.