It's not generally a good idea to play games with types like this, because they are masters at it and will see right through it immediately (learnt that the hard way..). But I would say that one thing that really did work, which I found out accidentally, is that they are so set on taking from you the thing that is most important to you that you can use it.
eg if I wanted ex to bring the children back late, I'd ask him to bring them back early [because I had "a social engagement and needed them settled before the babysitter arrived"]. Hey presto, I got what I wanted. And vice versa re if I wanted them back early I'd ask him to bring them back late. It worked every single time.
I'm not being flippant, especially as the stakes for you and your son are so high. But it is worth bearing in mind.
I would also counsel you to keep your true feelings entirely hidden. Do not reacte to him, do not let him see any emotion, do not 'appeal to his humanity' (he doesn't have any), do not try to reason with him. Maintain a kind of neutral response to him re 'that's interesting, can you explain that to me, I don't quite understand' instead of you fucking shit why did you do that you bastard . Don't let any sarcasm slip in. Act for your life. Don't respond to his hooks - or don't show any response, anyway. Also, let him talk; act, without being a doormat, like you respect what he has to say: 'yes, that's interesting', 'I see what you mean'. (You can only do this for short periods without vomitting).
Again, this is a risky strategy - but good to bear in mind.
imo what you must NOT do is lock horns. He will win, because he plays dirty and will use absolutely anything in the fight (as you have seen re your son). It will astonish you how low he is prepared to go.
You also need to set things up for future relations re your son could still get significant access time with him (unfortunately - though I could argue that his father is so toxic that any relationship with him is not in your son's best interests). I think you are going to need to take some kind of break from work while you fight this war. Get on to Womens Aid 0808 2000 247 at your absolute earliest - they are the experts and will know how to guide you through this and will support you practically and emotionally. Lines are busy during the day so a night time call would be better. I agree that something along the lines of compassionate leave could be something you could go for - I'm sure Womens Aid will have some ideas of how to go about a break of some kind without jeopardising your career.
If you get the right people behind you, it could be that time taken off work could be relatively short. You need your escape strategy, to get it in place; dot the i's and cross the t's so it is airtight and ready to launch without revealing your hand beforehand, or until it is too late for him to do anything about it. It's all in the planning.