I understand. I go through the same thing.
He can't not work forever. He will have to get another job at some point.
Send your son on his visitation with nothing. Just the clean clothes on his back. Take his picture and make sure you date it. If he comes back dirty, and in the same clothes take another picture and date it. No toys, nothing. Weigh him when he goes and when he comes home. Journal everything! When he starts paying you child support pack only a small bag with a single change of clothes. This will teach your X to provide for his son. Your X's vanity will do all the work for you.
Check over your son with a fine tooth comb when he comes home to you. If they are abusing him, you will know. If you see anything, take him to the ER and have them check him out. Take pictures of any scratches or brusing. Date them. Watch for behavioral changes in him. Read up on the signs of abuse. Journal everything! Get him into some kind of counseling as soon as he is able, so you are not the only one who sees these changes. Show the counselor all the pictures. The pictures will provide the court with proof that he is not fit to have visitation with his son.
It's a waiting game. If a counselor knows abuse is going on, it's their job to report it to the athorities. It is law.
Your X may surprise you. She may be abusive to him, but if she starts in on his child, he just may let her have it.
What you are learning now is what I have learned. Patience.
It's a sad thing, in our court system, that abuse can happen and people are still given rights. It's sad that we have to wait for bad things to happen to have proof that the possibility is there. It really makes me sick. It makes us feel like we are sending our children to the wolves.
One more thing. Tell your X you are watching him. Let him know very calmly and in a very direct mannor. Look him straight in the eye and don't flinch. Tell him if you see as much as a scratch on your boy, he will learn what it is like to deal with a mother bear. Tell him to tell his wife too. He may balk but don't engage in a fight. Don't explain why you are telling him this. Tell him you mean what you say and there are no excuses. Stay calm and cold about it. Do it when you are handing him over and then just walk away.
Keep your daughter away from the both of them. They do not have legal right to her and you do not give in to this by any means. When your son is gone make the time you have with her alone your time to do girl things together. Paint toes, do hair, go to the park, bake cookies and watch her favorite movie, whatever. Make her realize this time with your son away is her time for you alone. She will lose interest in going.
The next thing is the hardest and why we are put into these places in our lives. Give God a chance to protect your son. Let go of control and give it over to Him. You will be surprized at what He will do. :) Your son is a child of God first. If you pray for God to protect him while he is with his father, He will protect him.
You need to learn to use the system against him. Do only what you are ordered by the court to do and nothing more. Do not give in to any outside requests. Do not enable or feel pity for your X. He made his bed.
He will call you names, and be rude. Ignore it. Write it down. JOURNAL EVERYTHING!
Good luck to you. I will pray for you to be strong.
Keep me posted.