Glad to hear he is happier! That is definitely something.
Is his neck injury recent, or has it been bugging him for a while? I am definitely not a vet, but this does make me wonder if walking on the pavement is somehow jarring his neck. Outside of the hiking issue, does it seem like his daily activities are the same?
Before all this, did he mind walking on pavement? I had one dog who really preferred to walk off the pavement and would go to great lengths to avoid it whenever possible.