Sounds like a difficult situation. Your son is worried that his Dad has forgotten about him. That is one of the hardest things for a child to live with - no matter how loving and excellent a parent the one who sticks around is, many kids in that position will grow up fearing that they weren't 'good enough' for the parent who left. They weren't quite enough to be loved. Your son is already showing signs of that with the questions he is asking. And the effects of those feelings can be profound on children as they grow into adults.
You asked how you have the conversation with your son. Good question - and reflecting on that may help you to consider whether you are doing the right thing now.
I would suggest visualising the scene some years from now. Your boy is older. Quite a lot older. Old enough to want to know the details. And old enough to handle the details. So you're telling him.
His dad got into some trouble. He made some very bad decisions. And because of those decisions, he decided to leave.
So far, so straightforward. But now you come to the difficult part of the story. The part you've been worrying about telling your son. But the part where you either have to tell him the truth, or tell him a really big lie. The sort of lie that people can't just get over when it is discovered.
This is the part where he asks why his dad just forgot about him. This is the part where you have to tell him that his Dad didn't forget him. That - despite his poor decisions - he still tried to keep in touch. And you ignored him. You denied your child that relationship - however imperfect - with his father.
Does that sit right with you? If it doesn't - and it shouldn't - then you may wish to reflect upon the decisions that you are making now.
The guy doesn't sound like much of a dad. Or much of a human being. But it is not the right thing for you to do, to deny your son even the chance of having a flawed, imperfect long distance relationship with his Dad. Because when you do that, you leave him believing that his Dad forgot him. You leave him believing that he wasn't enough. That isn't right. And it isn't fair to your son.
My advice, for whatever it's worth to you, is to return those texts. Set up the phone call. Set up the video call. You can't compensate for the man's failings. But you can give your son the best possible shot at a relationship with his Dad. And, in doing that, you can let your son know that he is worthy of his dad's love.