I planned where I was going to go, told my boss (I work form home) and basically had 5 days to tell him I was going before I got the keys to my new place. He knew I was miserable (also emotionally abusive) and I had said we should separate many times before I actaully did it. i think its why he couldnt process it.
Anyway, kids were at school and I needed to go food shopping, so i told him I would only be buying enough until the weekend, because on the Saturday I was getting keys to a rented house and was leaving with the kids. He seemed to take it on the chin, I went out and for 24 hours he was actually quite nice.
Then the tears started, he was cooking dinner sobbing in the kitchen (wailing actually), the kids heard him, went to go and ask if he was ok (I had told him to go for a walk and calm the fuck down but he decided to sink a bottle or two of white wine while I was in my office working). He then announced to my eldest I was leaving, eldest was hysterical and took me hours to calm him down. It was horrendous and not the calm, mutual way I had thought we would tell them.
My point is you don't know how he will react. Make sure your son is out the house for severall hours, and tell a friend what you are doing just in case he kicks off.
Whats the plan when you tell him, will he leave? Or will you? Will he be able to leave straight away? You need to try come to an agreement on how you tell your son if possible, but that in itself it tricky.
I have never felt something so difficult than to actually get those words out, but I am so glad I did. I have now had 2 years of freedom from him (kind of he is still a grade A idiot and attmepts to abuse me from afar). A friend advised me to says those words into a pillow a few times, so they dont quite stick so much, it helped actually. Saying them outload instead of going over and over them in your head.
Keep it brief, don't engage in lengthy discussions about why. Good luck OP.