Was this just his immediate reaction and he came back and was rational when he calmed down? Or did he leave, never come back, and avoid you? Because if it's the first one, while it might not have been the best reaction, it's somewhat understandable. Hearing that your SO may be pregnant out of the blue, when you didn't have plans for babies happening would freak a lot of people out. Babies are a huge responsibility, and if someone isn't expecting even the possibility of one, doesn't have plans, and has other things going on in life, panicking is fairly justifiable. It'd be better to decide what to do with the relationship after the panic subsides, and after a pregnancy test to see whether or not you are really pregnant. If you are and he supports whatever decision you make in regards to the pregnancy, then he isn't a bad person, he just panicked. But if he tries to force you to terminate the pregnancy, to not terminate the pregnancy, or says he won't support you during pregnancy or the kid(s) that are born, then he is a selfish prick at best.

I honestly hope he has a really bad sense of humor. If not you need to have a serious talk about your relationship or just dump him if he would walk out on you like that.