Didn't spot this with regards the question above, but it's as I'd feared.
Best idea is to get in touch with maybe Citizens Advice or similar who should be able to put you in touch with the various support groups and maybe the council housing association as you and your kid need somewhere to stay.
Then when all is in place, just leave when he's not there.
Don't do it when he's around as that could turn nasty, but get out and maybe leave a note telling him his behavior has become unacceptable. Don't whatever you do give him any details of where you're going...!
There may well be a so called safehouse in your area where you can stay initially (support groups will know), but it's a lot simpler to do when you've got it worked out. A friend of mind (not the one in the previous post) did that, but this is learning from her experience.
Can't promise it'll all be easy, but unfortunately you're suffering the same problem as quite a lot of people and an awful lot don't do anything and suffer for years.
Why he does it - because it gives him power over you.
A third friend went through something like this and it turned out her ex was suffering from a genuine psychological illness. But from experiences elsewhere, most seem to be either ultra insecure or very nasty people. Even nice people seem to be able to tip over for reasons that I'm not sure even the best psychologists understand.
But whatever you do - start the process ASAP. Would hate to think of you going through the same thing as all 4 of my closest friends. Learn from them...