[fostermom]

Since your boy is 1 1/2 and not housebroken, it sounds as if he was a "backyard" dog and, as such, probably didn't receive much/any socialization or training. That might explain his fear reactions. We adopted our first rotti at about the same age and much of his behavior was very similar to your boy....not housebroken, clearly never been in a car, many fear responses. Leashing him to you is great advice to housetrain (and to let him understand what behaviors you want as they NEVER get to do undersired behaviors). Only give him freedoms when he is reliable...I would gradually let my dogs off leash when I felt they were trusworthy and would respond to my corrections...first only let off leash when you are with him in the same room, then gradually allow greater freedoms.
Lots of positive reinforcement builds confidence. Not rewarding fear behavior is also great advice...the absolute wrong thing to do is to pet (reward) him when he's fearful. Praise appropriate behavior. It does sound as if my boy was much more fearful than your boy as Reno was quite fearful of people...you could see that he WANTED to interact with us but was afraid to. Over time, Reno has grown into a confident dog who impresses all who meet him. I am sure your new boy will do the same. BTW, THANK YOU for adopting!! He sounds wonderful.