This is the WORST when this happens in a dog like labs. They have been bred to carry things around in their mouths (oral) so it's VERY hard to get them to stop. I've fostered a few dogs and have a lab and lab mix currently. This can also called lick granulomas (google to see if it fits your dog) a behavorial issue. I've even had to put an elizabethan collar on a dog so they would let a wound heal and not lick. It took awhile but when it was finally healed completely the dog left it alone.
Some physical things you can do are as you said, brushing might help. Lots of activity. Also make sure food isn't the issue. I have a dog that is sensitive to foods that were for allergy prone dogs. This is can take some trial and error, I saw Elisa had narrowed her dogs issue down to the dye.
If his skin isn't broken you can put something on his fur that tastes bad (talk to the vet about what would work best). When you do shampoo him, make sure to rinse rinse rinse and then rinse again. Did I mention you should rinse? That seems to help my chocolate lab. I would also try the shampoo SfromTX recommended. I'm going to look for that if our lab goes through another bout of itchies.
All the best, this is just not fun and you have to hit so many possible areas that cause the problem, top that off with the behavioral thing and it can drive one batty.