It’s so freaking annoying!!!
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More workout.
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Do not leave him outside unsupervised.
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Fill the holes with cinder blocks.
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Give him a safe digging area.
Increased mental and physical stimulation. Stop leaving unaccompanied children outside. If you’re outside, stay active unless it’s time to use the restroom. Add in a few additional walks. Nose work, puzzles, agility—all kinds of new tricks to teach.
They do things like dig because they are bored and want to entertain themselves.
Put the excrement in the hole.
That’s freaking genius!! 
Dogs normally dig holes in the yard, we’ll… because they’re bored… I would recommend creating or updating a schedule that contains a LOT, and I mean A LOT of more playtime and exercise. And when your dog do feel the need to dig, get a sandbox and encourage him to dig when you aren’t playing a game and in the backyard. Also, just don’t leave your dogs unsupervised.