Unfortunately, they love to dig. If you have an area where you CAN let him dig, that is probably the best solution. If he starts to dig somewhere you do not want him to dig, snap a leash on him or a rope and take him to the area you will allow him to dig and reward him in some way - praise or small treat. It is a natural instinct for them to dig and make a 'den' no matter their age or if they are spayed or not. It is about trying to get them to do something else. Trying chew toys - big strong ones - or bones can be distractions but always ask your vet or even a breeder for help with what they think might be appropriate for your dog, its size, age, condition, etc. Our dogs have lots of toys about the yard so they are less 'apt' to dig - although about every week, they have dug a big hole next to the house that we regularly just fill back in.