The best solution to your problem is going to take a bunch of physical labor on your part and some wire fencing.

Dig a trench from your fence into your yard two feet wide and a foot deep. Lay the wire fencing in the trench, attach it to the fence you already have and fill in the new stuff. Walk over the new stuff to pack it down.

Now when your dog tries to dig out, he'll run into more fence.

With luck, he'll eventually give up the digging.

Be sure he has enough to keep him busy while he's alone. If he gets bored, he might try something new. Stuffing a kong with peanut butter and freezing it is one ploy. Or a ball that you can put dry food into.

Your only other option is to tie him far enough from the fence so he can't dig under it.

Mary