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.
The dog may frequently dribble urine or pass smaller amounts of urine than usual and the urine is bloody or cloudy and is foul-smelling, plus, the dog may become suddenly thirsty all the time. Other symptoms include lack of energy and loss of appetite although these are not very common.