I have been looking on google for ways to talk to my son about his father and I came across something another mother has decided to tell her DD, simply it is 'Your father has some problems now that he has to deal with, and when he gets better and figures things out he will call and you can see him again.'
Here's the link
I'm going to use this the next time my DS asks about his father. Hopefully this can help you as well. good luck.
My thoughts are that you should just be a little careful about the phrasing you use when you speak to your son. I think it would be much better to say that his father MIGHT call and MAYBE he will see him again, rather than saying that he "will" call and you "can" see him. You are not in a position to promise your son that he can have a relationship with his father if the father isn't interested, and you may find that your son becomes resentful towards you if he feels that you have promised him something that then doesn't eventuate.
Remember in the author in the link is talking about a scenario that is a little different, her daughter's father had health and addiction issues and eventually had visitation rights taken away from him, rather than him simply choosing to walk away with no valid reason.