Pet stores sell repellemt to keep dogs and cats out of flower beds and such, maybe you could try sprinkling some all along the bottom of the fence until your dog loses interest in digging. He won't like the smell in that area so I doubt he'll stick around long enough to dig. While he isn't digging you could try installing new stuff in the yard to keep him busy. Maybe a few utility articles and such, or try giving him a nice big juicy raw bone to chew when he's outside.