I would definitely consult a dog behavorist.
Is is possible he is overstimulated? Too much activity and he doesn't know how to relax? Does he have interactive dog toys to make his brain work? (
Dog Toys - The Newest and Best Interactive Dog Toys Available From ActiveDogToys.com). He should work for his food/treats- A tired mind can often do more than a tired body. Have they ever done training with him. He may benefit from structured activity like rally or agility- Works the mind and the body.
Could it be separation anxiety? Some people swear by Thundershirts.
Does he destroy things right in front of them or as soon as they turn there backs? I know a few people that have used basket type muzzle to break the chewing cycle when they are home and can watch the dog. Never leave a muzzled dog unsupervised. The muzzle is slowly phased out.
If they can't watch him he needs to be confined/crated to stay out of trouble. He could accidentally ingest something poisonous if he is chewing everything up.