Listen mate, I feel you. My dad's got PTSD and he's done some things: let's see, he's strangled me (I'm not exaggerating here), pounded my head into the wall (well, this happens all the time, but I still find bloody scabs on the back of my head sometimes), forced me to bend my back to the point where (setting aside extreme discomfort) I literally thought my spine would break, etc. etc. etc.
Not sure what you mean by the belt though. If he just wanted to hit them with it, that wouldn't be so bad. Yeah, it hurts so bad but the sting will go away after a while, and then after that, the bruises will disappear too.
I'm not afraid of him anymore, I'm afraid for him. It takes every ounce of effort to not snap back and fight. I'm taller than him and I know everything about him: I'm just afraid that one day I'm not going to be able to hold it in any longer. Don't fight it kid. It'll land you nowhere good.
Wish I could tell you more, but I don't know anything else about your situation. Best of luck.
