My dog will shake and growl when approached quickly and or pet him quickly?

First off, we're don't abusive him AT ALL. I'm 14 and have two very loving parents. However, we got him from a family because they didn't want him or something, not sure, but he didn't have any papers on him and he seemed a bit sketchy. He's a male shih tzu, not positive though. He's not neutered. I think he looks at me as the dominant male because he moves his head away when there is eye contact and he bows his head when being pet, but still shaking a lot... All the other dogs are very loving while he doesn't express much ''love'' at all. What should I do and what do you think is going on with him?