You have to be careful with the electric fencing. In our township, it's illegal to have it. You could use concrete pavers over the spots where the dog is digging. If it's a chain link fence, some well placed metal tent stakes will keep the dog from being able to squeeze under. Or you could try mouse traps. Set them and then cover them with a paper towel. As soon as he touches it, it will spring the trap. It makes a scary noise but won't pinch the dog. I know because I used them on our table surfing dogs! If he's a nervous dog, it should work.