The border traverses a variety of terrains, ranging from major urban areas to uninhabitable deserts. Approximately 350 million legal crossings occur annually, and is the most frequently crossed border in the world. The total length of the continental border is 1,954 miles (3,145 km).
Only a veterinarian will know what's wrong with your dog but usually stumbling, wobbling and falling down is associated with neurological problems, especially if there aren't any other signs. I would really take him to the vet as soon as you can.
Limping is a symptom associated with tumors in or on the bones, or in the brain or central nervous system. If you see suspicious symptoms accompanying your dog's limping, such as dizziness or disorientation, or if you find any odd lumps or masses, get your dog to the vet as soon as possible.