I know what I’d say to me

Fireflyaway

If he suddenly and completely changed after the head injury I'd be inclined to be patient (whilst keeping safe and remembering this is not your fault) and see what a professional medical practitioner says. If it's really not his fault that's one thing, but you hint that he was a bit off before the accident happened. That suggests he managed to control his temper a bit better previously ( probably to get you to a situation where you settled down with him and would be less inclined to leave) but has now let the mask slip. I'm of the opinion he probably won't change. The damage this could do to your son is also worrying. My husband has depression and it took a long time for him to confide in me that his dad was abusive. I'm convinced he has some sort of trauma related depression because of it. He can still remember hearing his mum being verbally abused and beaten up. That's an awful thing to comprehend and it will never leave him. Set your son a good example that abuse is not acceptable and you won't tolerate it. You don't have to tell him in words. He will see because of your actions. 
I'd recommend you don't let your husband know you are planning to leave. Put some money aside and as others suggest, call woman's aid. The sooner you do it the easier it will be and the quicker you can build a happy life for you and your lad. I tolerated abuse because I hoped it might stop. I could still see good things in my partner, I was scared of the unknown if I left and I was worried what others would think. I was embarrassed. Looking back it was silly. The only person in the wrong was him. Even if I did do annoying things (don't we all!)  violence is never acceptable. Ever. You are justifiably sad about this. Not only are you being abused but you are mourning the loss of the future you had planned. You still have a future, it's just different. Once you get past this episode imagine what a great life you could have. Definitely don't let buying a house put you off. Just rent somewhere to begin with. I'm renting and it's perfectly fine. Step by step you will get there.