I always suggest making a sand box for them. Put it some where in the shade and bury treats in the sand (slightly large treats so he does not get a mouthful of sand). Entice him to dig there and praise him when he does. If he is digging some where other than the box then take him to the box and get him to dig there and praise when he does. This requires you to watch him whenever he is outside so he never gets the chance to dig out of the box. If there is one spot he really likes to dig either put the box there or is that is out of the question, put some chicken wire under the dirt there. I have also heard that is you put poop in the holes they dig that they will not dig there any more, this of course also requires that they dig in a specific spot. SO I would make a sand box for him to dig in for sure.