He is obviously someone who can compartmentalise very well. Whether that's to protect himself or to distance himself you can't be sure.
If I were you (and I speak as someone who hasn't been in the same position), I would want to make the most of the contact you have now because it might be your only opportunity. I would tell him about your life - kids, what you do for work etc. I would then ask him about his life. It's difficult with Covid but I would maybe suggest meeting him in person for a walk just to chat. I wouldn't bring up anything major during that meeting, but meeting in person would likely be more natural for you both. I'd then see how that goes and take it from there.
I understand there are questions you want answered, but I think you need to do a bit of groundwork first. And when I say you, I mean both of you (mainly him).
It does seem very unfair that he seems to be the one who is unsure/contemplating the contact at this point. As a parent he certainly owes you that at the very least, but, I hate to say it, as he was able to walk away and compartmentalise that part of his life, it is possible that he could keep the lid completely shut because he finds it "easier". If you want answers from him, approaching that too soon might scare him off. At the moment you are just this "idea", not a real person. He may need time to get to know you before you feel able to open up a dialogue about the past. I will say that that doesn't seem fair, he owes you much more, but his actions up to now indicate that he doesn't necessarily go by what's fair or right and if you want answers then you might have to play the long game. If you don't want to play to his tune then that's fine too. It's just a case of working out what you want from him and then the best way to get it.
I really do hope it works out for you.