My father died when I was one year old...My mother re-married when I was 14 to a guy who hated me...so that did not go well for me...
Now your situation is different. And i am sorry to hear your ex is being a DumbA** to put it simply.
What you need to do....is one day just you and your 18 year old son is sit down.
I want you to have all your bills in hand...
Mortgage, electric, gas, Car, Insurance, etc what ever you have to pay. Average food bill.
Then I want you to show him how much you make....NET.
Show him all the money your Ex-husband has paid to you since the divorce.
Show him the COLD HARD FACTS.
Then go on, your ex-husband is not a bad guy that it just did not work out for you both, just like when he breaks up with a girl friend.
Tell him you did your best to give him a good home over the years, that life could be a whole lot worse, that you ahve not demanded the money from your Ex-husband, the money that was for HIM (your son)
You tell him... that you will amke a deal with him. That for each check your ex-husband gives you for Child care that is for Him, that you will give him $200 of it, that he can go with you to the Bank to cash it when you get it.
Let him kow the TRUTH. Also you tell him he can live with you as long as he wants, but the minute he gets violent he will have to go live with his father.
Write it up in a contract if you have to.
You see I had a older brother that was violent after my father died, he felt he was "shorted" and turned to drugs and Violence, even so bad he used to hide his drugs in my bed and I was a 5 year old kid, My mother ended up kicking him out of the house, she didn't want to, but she had to protect me, that brother ended up dead in a Bar fight some years later.
So you need to cut this path of destructive behavior, give him the facts, stop being the nice guy, Show him what you ahve done for him, show him what his father has done for him, Put it on paper, give him real evidence. Then let him question his father. Let him see the no money comming from him, let him see that it was YOU that raised him.
he will figure it out some day, trust me, he will.
I also hate when parents use children as pawns. I know other couples like this and makes me sad and angry. I told my wife if we ever got divorced that I would remain civil towards her and that our children would always come first.
Just remember everyone has a point of view in a situation. What ever it is that your son has seen has influenced how he is. Coming from a divided house often means that parents do not act together with unity when making decisions for their children.
Your eldest may also have many other issues that have not been addressed. He may blame himself, you or your husband for the divorce and depending on how life became after the divorce he may blame that on said person(s) too.
He may not want/need your husband to be a second dad or a friend which he may feel was pushed on him.
He might also just be doing the typical rebellion thing.
If you are going to talk to him anyways I think you should ask him for his point of view and tell him that while his rebellion makes him stand out now it can affect him in his very near future and that you are worried about him and want to be there for him.