I wouldn't be so quick to decide that he's been abused, kicked, or otherwise mistreated. Some dogs are naturally shy. And some very small dogs tend to have a natural aversion to being approached by someone so very much bigger than they are. Tango tends to back away from me when I approach him standing. But if I squat down, and get more on his level, he will most often run right to me and jump in my lap.
So that's what I'd start with, don't try and approach him standing, don't walk towards him, don't tower over him. If you want him to come to you, get down on his level. Also have treats. If he begins to start associating coming to you with yummy goodness, he'll be more inclined to approach you even when you are standing, but certainly when you're squatting.
Whether he's been abused or is just shy, it may not be a particularly fast fix, so be patient.