I'm sorry that your question wasn't answered in a timely manner. Obsessive digging is a frustrating and vexing behavior and the answer to it may be elusive. Digging may be a nuisance but we must understand that it's an innate trait for many dogs and such traits may not be amenable to correction without altering the quality of life of your dog.
Some breeds may dig holes that provide a cool place for them to lie. Some dogs use digging as a way to escape confinement or the digging represents an activity similar to destructive chewing that occurs when young dogs are left alone without sufficient stimulation. When dogs become house pets, they often need to leave natural tendencies behind, such as digging, if they are to be good home companions. Most dogs have little problem with this compromise as long as they have sufficient stimulation elsewhere in their lives. Some dogs, however, may continue to dig despite other adequate stimulation, to escape or simply because digging is fun. Understimulated and underexercised dogs may dig as a recreational event. The prognosis varies considerably with the underlying cause. A young dog or intact male with a strong motivation to roam may see digging as providing freedom; these dogs are very frustrating to control. For these cases, keeping the dog indoors in a safe, destruction-proof area or providing a confinement area where the dog is unable to dig to escape may be the only viable alternative. Environmental enrichment is most indicated for those dogs that dig because they have no acceptable alternative. Whenever the pet is left outdoors unsupervised, it is important to attempt to provide an appealing alternative activity to distract and occupy it. This distraction may include large balls to push around or wooden boxes and ramps on which to crawl and explore. Large rubber toys can be stuffed with treats, tied to ropes and suspended from tree limbs for some dogs. The success in enriching the environment is variable, however, and may be negligible for some pets. Increased activity, such as vigorous physical exercise (fetch, jogging, speed walking) provided two or more times daily, may help reduce the amount of time digging. Another option is to provide a sand/soil digging pit with partially buried toys and "chews" to encourage digging in one area instead of many. Adding another pet may be helpful but you might end up with two pets that dig and therefore twice the damage. When dogs are digging to create a cool respite, they may stop if given a cool, shaded area or a wading pool is provided where they can cool off. Dogs that are digging as a response to fearful stimuli may enjoy the comfort and security of a doghouse or other forms of shelter. For some dogs, confinement in a secure pen or run may be the best treatment plan.
It sounds as if you've already provided suitable exercise and he's certainly not ignored. There's likely to be more that can be done but you'll need a specialist behaviorist to come to you and examine your dog's environment and the dynamics therein to be more specific with a treatment plan. I'm necessarily constrained by distance. There's no substitute for a "hands-on" examination when it comes to behavioral disorders. In the UK you have apbc.org.uk as a good resource for such a behaviorist.
Please respond with further questions or concerns if you wish.