If you can in the place where he is digging, place those large stepping stones. You might want to keep extras in case he begins to dig around the perimeter. (Hopefully you get this under control first).

What is happening, he is relating or has learned in his past, that freedom comes from under the fence. Once you block the "freedom" and he can't get out, then he can put it together. If you can't dig, you can't get out.

When you notice him digging, a stern NO, then give him play time, a chewy, or some kind of reward. Once he makes the connection, it should stop or at least be very minimal.

Good luck.