Agreed. If this was a completely unplanned pregnancy, and most specifically, if precautions were being used to prevent a pregnancy, the shock (I can only imagine) would be pretty insane.
It is a big jump to go from "planning on getting engaged" to starting a family together. You said yourself, he overreacts and flies off the handle at things... this is quite possibly the biggest of things, knowing him and his tendencies, this should have been somewhat anticipated.
He's probably in shock, again, especially if preventative measures were taken... if not, then you were playing with fire to start with. But he probably got his with all the anxiety and pressure that comes with being hit with it... does he make enough money? is he going to be a good dad? is he ready? Did he react well to it... doesn't sound like it... but from what you said in your post, it sounds like his typical response.
You are likely going to need to give him time to process the information, calm down and put his big boy pants back on. To be honest, it probably won't feel real to him until he sees the first ultrasound, or maybe even the baby move inside you for the first time.