drastic I think it may benefit you to contact Women's Aid on 0808 2000 247 and your local children's services. They will be able to give you much more accurate advice relevant to your personal circumstances about how to deal with this is the longer term.
In the shorter term, although I know it's frightening, I think it's important to stick to your plans. You took the hardest step of leaving him 18 months ago so you can do this, you know you can. Bravery isn't the absence of fear, it's the determination to continue with something despite that fear, and you CAN.
You need to have a chat with your DS about all this. Although it's important you don't slate his father, that doesn't mean lying to conceal his bad behaviour. He &has^ behaved badly and it is not your job to conceal that from your DS or to make excuses for it. Your XP is accountable for his own behaviour and he will have to explain it himself. If you minimise it or deny it, you are teaching your DS that this behaviour is ok as long as it's kept quiet, or even worse you could end up painting yourself as the bad guy.
You are not 'bad-mouthing' your XP if you tell your DS something along the lines of "your father and I are having some problems at the moment. I feel that he is bullying me and I am taking steps to make it clear that I will not put up with having to change my plans on a regular basis because he wants to change days or events, nor will I put up with name calling or being physically hurt." You have not said anything untrue, called your XP any names or made up any bogus explanations as to why your XP behaves like he does.
Your XP probably will bad mouth you, sadly. All you can do about this is try to get your DS to talk to you about it and set things straight. So if your XP says "mummy won't let me see you today" you tell your DS "daddy is mistaken. He couldn't see you at the time we'd arranged so I had to make other plans. He can still see you until tomorrow." Do not try to explain your XP's behaviour or call him a liar, just concentrate on showing your DS why you're behaving in the way you are and why you feel it's in his best interests. It will often feel thankless and unfair, but by the time your DS is grown up, he will see straight through your XP and he will have wonderful communication with you, his mum.
You can do this. 