Since you don't know his previous living arrangments, it's hard to say.
Unfortunately, dogs that have not been socialized well can have a tough time adjusting to being in a family, living indoors and the noises of a household.
The dog may not have been abused at all but is shy because he has not been touched much by human hands.
If he is eating and going to the bathroom ok, you will probably need to give it some time. I suggest that you give him a quiet place to lay where he has access to her food and water and that you quietly step in every now and then and offer treats. Approach him slowly and quietly and, if he seems fearful, leave. Once he begins to realize that you bring treats, you can try sitting alone in the room with him and offering the treats to him but make him come to you.
Another tip is that you should never approach the dog from the front. If you reach for the dog, reach from the side so that you can avoid some of the "head shyness."
This is going to take some time but these dogs that are poorly socialized can make loving and affectionate pets if you train them gently and easily. He will soon be used to being in doors.
Please let me know if you have more questions.