Do you actually walk your dog or just let him out in the back yard?
I would suggest long walks 2-3x a day, you can monitor your dog's output and he will also get some mental stimulation. We don't have a yard, but yet we never had inappropriate bathroom habits, only if our dog had a serious illness.
Also, who recommended the vit C, the vet? Just asking because generally dogs do not need supplemental vitC, they make their own.