I like the idea of asking your ex to call your son once a week. Do this without your son's knowledge. Either he will make it clear (your ex) that he doesn't care to do this or has "compelling reasons" why he can't (he doesn't want to), or else he will agree to try. Again, say nothing, but when the time he agrees to call comes, he will call or he won't. If he does, that's a nice surprise, for you and your son! Let them talk, and see what happens at the same time next week. In books on recovery for kids who grew up in alcoholic families and have a hard time knowing if people are "safe" (that is, reliable) or not because they grew up surrounded by unrelible, unsafe people, this pattern is called "share/check, share/check" That is, you try sharing a small thing with someone. In this case, it would be that your son misses his father and could use a regular phone call. Then you stand back and see what happens. If he SAYS he wants to help, and that he will call at a mutually agreeable time, and then he CALLS, then that's great! You can, at some point in the future, if the calls remain consistent, share more about your children's needs and development with him and perhaps hope to get some more appropriate support for them. But if he says he can't manage to call because this week is so busy or whatever and tries to find reasons to STAY out of contact with a son he hasn't been in touch with in two years and who is asking why he is completely gone from his life, then, sadly, this is a man who is broken in that way and surely won't be able to make the far greater effort to come visit the kids.
Unfortunately parents like this have a way of occasionally drifting into their kids lives at the worst possible moments, so it won't exactly work, if he can't manage any kind of simple consistency, to tell your kids "look, your father is just broken in terms of his being able to love and connect with great kids -- that is him, not you, and for now I and the adult friends around us are your family and we are hear for you because we can be -- your father would be if he could -- it's very sad he can't stay connected with you, but it has nothing to do with you," because once in awhile he probaby will call or send a card or turn up on your doorstep with balloons. So, "he's far away, I'm sure he would come if he could and I am sure he will come when he can" are also true, and simpler nd safer.
Good luck -- I hate it when parents are so broken then can't meet their kids needs at even the simplest levels -- so sad for both the child and the parent -- think about how damaged someone has to be before they can't meet even the most basic needs of their own child! I know for me as an adult healing came like lightening when I realized that anyone who couldn't love a great kid like me and wasn't proud to have me as their child was really very damaged, and I was able to feel compassion for my parents rather than anger. I've felt much lighter ever since! Anything to help your son know HE is just fine -- a fabulous kid -- and that circumstances beyond his fathter's control are preventing his father from staying connected, is both true and good for your kid's self esteem.
M.