It just sounds like he needs to be exercised more. The collie is a very high energy working breed as you already know.

And if this dog doesn't run he has to have something more interesting than those varmits. And that is tough to replace.

I am not sure if a toy is going to fix it if he has other dogs to mess with. He is just going to have to be worked more of kenneled until he knows not to dig.

Catch him digging, let him know you are not pleased and kennel him for awhile were he cant dig and is no fun to be. After a couple days of hard time he will get the picture if you keep up on it.