It sounds like he might have some confidence issues. Confidence is just as big, if not bigger, with dogs as it is with people. A lot of praise when he does good and nothing negative will help. Having people, even yourself, throw a treat or give a toy when they come into the room will begin to show that people are ok and are not scary. Taking him to a trainer will definitely help because they’ll be able to give you the tools that are best suited for his needs. We kind of dealt with the same issue with both of our pits. Our first was frightened of some people, but it was more of a nervous issue. So it was lot of socializing, play time and treats before she was able to be comfortable with new people. Our second was afraid of objects, literally anything and everything. So again that was a lot of treats, praise and play if she walked near, sniffed, nibbled and eventually got to the point of being next to something. Its going to take alot of patience and work, but I’m sure it’ll work out for you. Good luck!