Hard to tell (or rather to guess accurately) from the angle of the photo. Looks like a true Heinz-57 to me. Maybe some shepherd in there, possibly a little beagle, maybe some lab or hound of some kind.
Have you taken him to the vet or local shelter to be scanned for a microchip? Even if microchipping isn't something that people in your area routinely do, lost pets can end up in the strangest places and it costs you nothing to have him scanned. All the local vets, feed stores, and shelters would be a good place to put found dog fliers too. I know if my dog were lost they would be the first places I'd contact to see if anyone had turned him in or seen him.