The most common cause of foot licking is an allergy problem. Dogs can have allergies to pollen, grass, dust, food, and can even have allergies to fleas. If you suspect this, you should discuss the possibility of using an antihistamine with your dog and changing her diet to one that is low allergy with your vet.
Paw and foot problems can also be caused by a thyroid condition so I suggest that you talk with your vet about blood work and urine cultures to rule this out.
For the discomfort that he already has, you can clean his foot well with a peroxide or betadine wash (1 part peroxide or betadine to 4 parts water) and then dry it completely, even between the toes. Then apply some neosporin ointment to any areas that look affected. If he continues to lick, you can then slide a baby or child's sock over the foot and tape it lightly at the ankle. This will discourage the licking and give the foot time to heal.
Let me know if you have more questions or you want to discuss this further.
