Since it's only on his feet, maybe he is allergic to grass, your carpet, or maybe your floor cleaner. Some dogs lick their feet out of boredom. Does he get enough exercise?
You could try wiping his feet every time he comes in from the lawn, but that is so difficult to keep up. You could try Helen's bleach soak. She swears by that treatment for most any foot problems. I forget the recipe, but someone here will share it.
I use a scaled down version of her soak. I put water in a large bowl and add a cap full of bleach. I put one paw in at a time for just a minute or two and then pat dry and dust with Gold Bond Baby Powder.
