You need to put your foot down with your son before things get out of hand. I have a brother who is 12 years younger than me. He turns 30 this month and is just now moving away from my parents house. My mother has let him walk all over her for years. If he got in a bind, she bailed him out. He didn't do anything around the house to help out and she did everything for him. He needed cigarettes, she bought them. He needed gas money, she gave it to him. He was late on a car payment, she paid it.
She did this to the point of her and my father loosing their home. She would pay for things to help him out even if she didn't have the money.
He has had many jobs over the years but he is lazy. If he didn't feel like going to work then he didn't. If he went out with friends and got home late and was too tired to go to work, he didn't. I blame a lot of this on my brother but also on my mother for enabling him for so long. He has decided to move out with his girlfriend now and my mother is beside herself. She is afraid he will get out there and find out he can't make it. I told her he was 30 years old and it was time for him to man up. This is what you need to do with your son. Tough love...they all need it one time or another.
My brother also "pretended" like he was going to hit my mom at times. My mom is about 5'4" and weighs 100lbs wet and he is 6'2" and weighs about 200lbs. She does not need that stress and neither do you. You need to sit your son down and give him the "house rules" and tell him if he doesn't abide by them then he has to leave. My daughter is 18 and graduated from high school last week and I sat her down and told her that she would have more freedom now that she is an adult but as long as she lived under my roof that she had to listen to me. She has a job and has been paying her own car payment for the last 1 1/2 years. That is how they learn responsibility. I told her I would not charge rent but that she could help out as needed.
Your son will think you are against him and maybe threaten to go live with his father and if he does tell him that is fine. You don't need the added stress in your life. Besides, you have your nephew you are raising and if he sees that your son isn't doing his fair share then he will grow up thinking he doesn't have to do anything either.
As for grabbing the phone and telling your xh how he was doing, I probably would have done the same thing. I am not saying it was the right thing to do but I understand how you want help in dealing with your son. If your xh was any kind of man then he would have gotten back on the phone with your son and told him he needs to listen and respect you. For him to act so childish shows he is no more of a man than your son. They are playing games with you. Stop it now. I know you want a friendship with your xh but sometimes it just isn't meant to be. Heck, my stbx told me one time he was hoping we could be best of friends again one day. Umm, no thank you. I will be civil to him for the kids sake but that is it.
Good luck with whatever you decide to do with your son. Just remember, even if he gets mad he will always love you.