We have a digger here, too so I feel for you. Sammy (aka "Pig Pen" the dirty kid from the Peanuts cartoons) digs holes b/c he's a Pyr and that's what they do
but also b/c it's summer and the ground temp is cooler than the air temp (even here in the cool Pacific NW). So if Mack is finding summer to be hot, he may be digging more to find his own form of AC. My patient husband just filled in Sammy's latest earthmoving project b/c it was about to undermine the foundation of our garage. DH filled it in with 4"-6" diameter rocks and Sammy isn't amused, I'm afraid. However, he hasn't tried to dig there again yet. The rock method also worked wonders with Cara, our Pyr/backhoe from years ago. Our pups have loved to dig along foundations or under shrubs or trees, so rocks have looked good in those locations, too. If he digs in the middle of the yard...well...
.have you considered turning his projects into koi ponds?