I see you called all the vets in two counties, but have you called any of the pounds or rescue groups? Have you checked Craigslist. Someone who thinks they "found" a big dog might want to find it a new home by listing it. Check the local papers, grocery store and laundromat bulletin boards. Post flyers at your house as well as where you trailed him to. In my area people put up BIG posters protected in plastic so they can be out in the elements for a while. If you don't have a photo of him use one from on-line that is similar. List breed, picture, name, phone number.
I am hoping you get him back,
Jenny