Words can not express how your question has touched me. I was a single parent for many years, and the situation was the reverse of yours. My ex spouse, the mother, was playing your ex husband's role, and I was in your role.
I know it hurts, especially when you have no one else.
But as single parents from broken families we have to realize a hard truth.
The hard truth is, that at some point, male children need to identify with the fathers, that is part of how they grow up to be men.
I know you mean well by telling your son "the truth" but sometimes the truth hurts.
your son is hurt, but he is not able at his age to acknowledge he hurts; instead he stops off at the emotion we call anger.
This hurts because, from psychology, he is identifying with his father and trying to assemble a positive image of himself as a man.
When you tell him the "truth" you are in essence, unknowingly (unknown to you and unknown to him), criticizing his image of himself as man.
This is why most psychologists and relationship counselors advise their clients who are separated and divorced, to never say anything disparaging about the other person, no matter how bad they may be.
In the best situations, if one could find redeeming value in the other, and it comes up in conversation, that you should emphasize the positive. For example: "son, your father loves you and wants the best for you, he just has a hard time showing it."
We often recommend that if you can not find redeeming value and say anything positive, it is better to say nothing at all to the child about the other parent.
The other hard truth is, that your son is now old enough to make his own decisions, and legally, at age 18, he is an adult and free to move anywhere he choses.
So what would happen if suddenly the dynamic between you were to change?
You can cause good things to happen, but it will be hard. It is a forum of tough love, and yet a level of letting go of your soon to be man.
What would happen if you were to take your son to dinner, at a nice restaurant. During the dinner, you bring up the subject of his moving in with his father. But your tone is different now than it has been in the past. You can design the scenario anyway you like. But perhaps it is like this:
Son, I understand that you need to do this, move in with your father. I want you to know I understand that. I am not happy about it, because I am alone, and you are all I have. But I understand that you have a need to do this, and that you want to do this. So let me say this: I love you and I will miss you. I just want you to know, that I would appreciate an occasional phone call, and that you are welcome home at anytime. You can spend th night, the weekend, what ever you like. It would be nice to meet for Sunday brunch (diner, etc) (every Sunday or every two Sundays or once a month). Please do not hesitate to call me at anytime if you need anything at all.
This will be hard to do for you. you are respecting his decision as a man to get to know his father.
in the end, the father is responsible for his relationship with the son. If he messes it up, his son will realize it, and he will be home again, soon.
While he is gone, you need to be prepared to stand by him if he needs it, and if he calls. You need to keep the channels of communication open by sending emails, and birthday cards, and other holiday cards. do not forget birth day gifts and holiday gifts.
What you need to do is change the playing field, the dynamics. Do the unexpected. Acknowledge his need to move on and to get to know his father. let the father be responsible for his own relationship with the son without editorializing his fitness as a father to your son. let your son know how you feel without saying anything bad about the father, and let him know you support him.
You will see what a change this will make.