This was the absolute hardest thing for me. My ex wasn't abusive, though he is selfish and kind of an asshole. I'm familiar with your thread on PaP, and it does make me nervous for you.
We'd been having problems for a LONG time. One day I finally told him that I was thinking of moving in with my mom. He was SOOOO upset. I honestly don't even like to think about it. It was pretty much the worst day of my life. I'm not telling you this to scare you, it's the truth.
In the end, I ended up begging him to make huge changes with me, and I talked about moving and enrolling in school and doing huge, huge different things, and he agreed he wanted to try that. But within a few weeks, he'd changed his mind and said that he didn't want to go with me. (I was already enrolled and ready to go in the fall.) But we were going to live together and co-parent until school started. By May (this year) he was asking about when I was going to move out, because it wasn't working out. So I made arrangements to move in with my mom over the summer, and when I actually moved out he was mad all over.
I've been living in my mom's house for three weeks now, and it was the best decision I ever made. I am SOOOOOO much happier. I'm so much more relaxed. The kids are more relaxed. Even though I'm living with my mom and stepdad, they are ten million times easier to live with than my ex. In August I will be moving into an on-campus apartment and going to school full-time, and I cannot wait.
Honestly, I don't think time of day or words really matter. It is going to be hard and suck no matter when you say it or what you say. I regret that the kids were at home when I had the conversation, because they saw their dad cry and it really scared them. If you really think he's going to be physically violent, I'd just move out one day and have him served with divorce papers after you are gone. Or perhaps have someone you can trust with you when you tell him. There are domestic violence hotlines that could maybe help you work out the specifics.
Maybe the wording/scenario is something you can work out with your son's psychologist? Or do you see someone for yourself that could help you devise a plan?
*hugs to you* I know this is SO SO SO scary, but you will be so much happier when you are free.