Disclaimer: i know you have been doing all you can to keep you both safe and have endured a lot of abuse and are still, and that it took a lot of bravery to leave, so please read and understand im not saying you havent done enough to protect him or yourself, just that its not over yet
as much as you have been thru, that you have and are both going thru, its time for you to really, and i mean really stand up to him, start by saying NO, and keep saying NO until you actually feel confident in saying NO, say it and mean it
nothing has changed in your life he is still abusing you both and its now really effecting your son, you need to be his protector in this, the visits are detrimental to his mental health so you need to make it stop and stop today!
start by saying hes not coming and if dickhead turns up you will call the police, even if you have to send your son somewhere for the night to be safe do it,
get your son in to see a therapist or counselor or someone who can help, but also with the idea of help in a court case, if your son agrees for them to tell a judge the effect his father has on him.
maybe some help for your self too, as he is still ruling the roost in your house, so you need to get a little help with your already stiff backbone and make it steal plated, as the abuse you are still getting needs to be stopped and hes wearing you down with every demand, so get some support and a way to vent.
make sure you refuse any verbal contact with your ex dickhead, text message or email only from now on, so you have evidence of his abuse, same with your son... cant be he said she said that way!
then get a lawyer and get to court and get this visitation stopped, yes it might cost more than you can afford for a while but the benefits for both you and your son will last a lifetime, and hopefully give you both some much needed respite from the abuse you are both still suffering from, and your son will know in his heart you have his back and will go to the wall to save him from the abuse he is still suffering from..
again you are doing a great job but unfortunately for you both there is still some work to be done to get the peace and safety you both deserve