That's fairly normal. Lots of dogs bark a lot at the door, because they know the door opening often means a new person is entering. If he is scared, it probably means in his past home, someone scary often entered. How lucky he is to have a new home where no one that comes in will hurt him.
I would condition him to it by exposure therapy.
You'll start by going to open the door, but stop right about or just before the time he usually reacts. Keep doing this over and over, until he doesn't react, and then do a little more over and over, until your able to open the door all the way without him barking. Treats can and should be given as well when your doing this, especially when he doesn't react. This will take place over some time.
For example, if he starts freaking out at soon as the door handle turns, go over and and touch the handle just enough to make a noise, over and over and over for a while. If he continues reacting and barking the entire time, do this for some time every day until he stops. Try and get someone else to give treats while you do this at first, and then just when he doesn't bark. Once he doesn't bark at the handle noise, you'll progress to the next step, turning the handle all the way, and repeat the same steps.
If touch the handle make him panic, you'll need to start lighter, like just going to the door. The goal is to get him used to it, and it's a lot easier if he never gets all the way to panic mode. Comment with any questions.