I have exactly the same problem with my Border Collie/Something X and he has taught my other dog to do the same................................ Bless! What I have found that helps is if you give him somewhere to dig, ie. his own patch, somewhere where it is not too noticeable, let him be there when you prepare the ground for him, if he is food oriented bury some goodies all over 'his' patch of ground along with a couple of favourite toys and things and make a game of it, if you see him digging in places he shouldn't, take him over to this piece of ground and again bury something there and let him dig it up, if he is as intelligent as my boy Harvey is then he will soon get the idea, he does sometimes 'slip up' but I just have to jog his memory occasionally, do you take him for plenty of exercise? and could you perhaps do some agility with him, these types of dogs need a lot of stimulation, both physical and mental.