I agree with AnnieBrooke, you should definitely seek counsel from a child therapist. This isn't something that you should have to deal with on your own. That's a lot of responsibility for one person. It's unfortunate this man decided to take on the role of dad and then bailed. Someday you two will learn how to trust again, but take it slow. Are there any Grandparents that can come support you two? Even though children love and need their parents, I think it helps them a lot to know that other people love and care for them. Have you ever heard the phrase "it takes a village to raise a child?"
You shouldn't have to raise a child by yourself. I'm not saying you need a husband or boyfriend, but you need support. Good luck!
It will get better as times goes on the asking will become less, its sad isnt it for children ,perhaps once you have a new relationship he will feel happier , have you any male relatives or friends could play ball games and mentor him ?
Usually I think some transparency is important because other people will tell him things and it would be better coming from you. The sperm donor is easier to explain than the take-a-powder 'daddy' who the child might actually remember later. With the sperm donor, much later than now, you could tell the child that you made a bad relationship mistake and it ended about as soon as it began, but one super-great thing came from it, and that was him. As for the daddy-ran-off, I think for the sake of your son I would try to get back with him if he's around, if only so your son could see him, but if he won't do it he won't do it. All you can say to your son is that he is gone, and that it makes you very sad but that some things don't work the way we want them to in life.
You might also talk to a children's therapist for help with the kind of language to use.