It sounds like an allergy, although it may or may not be more involved. You might ask your vet to run a blood panel and a urinalysis to get an overview of your dogs internal health status. Check your dogs thyroid levels as well to be sure he isn't hypothyroid as that lends itself to skin issues.
Pemphigus is an autoimmune disorder that can also cause feet lesions similar to what you are describing, which your vet can rule out with testing.
Weekly organic oatmeal baths, a grain free home made diet and a supplement to boost your dogs internal immunity would also be valid considerations.
A pair of cotton socks would help to keep allergens off your dogs feet so they can heal may also help.
Be sure your dog and home are free of fleas.
If you'd like to follow up on this with me, I am glad to be of further assistance.
Dr Carol Osborne, DVM
Lisa,
I have a Pyrenees/yellow lab mix. She will be 2 in February. She has the same problems as your dog. At 1st my vet was giving her steroid shots every 4-5 weeks but said that he did not want to continue to do that since she was so young. He also told me to give her benadryl but I told him I did not like keeping her drugged up because all she does was sleep. He has took blood test, looked for mange, took other test but can't find nothing. I have been feeding her homemade food for 6 months now rotating the meats I put in it. He told me that food allergy test are very expensive so he had me to only feed her chicken with the grain free mix(Sojos or Pet Mix).If you know of a better grain free mix lplease et me know. He said we would start by elimanating 1 thing at a time to try to pin point it. So far she is doing good since I took the other meats out of her diet. I started her on the chicken only about 2 months ago.