Two suggestions;
- treat him for yeast infection too (try smelling his feet see if they smell odd, any cracking of skin, or discoloration) So, vinegar bath to the feet or other "home remedies" for thrush.
- go to a farm store where you don't have to fuck around with a vet and get one of the "hot spot" / infection / fungus sprays. Any place that would have cow vitamins and crap like that. (Watch out for horse-chicks!) It'll be a squirt bottle or a gel bottle. I use "Purishield" brand (photo of a horse on it) on my cat as a cleanser for chin acne and foot infections from crappy claws breaking all the time. There is another brand that is good (i forgot which) that has "activated water" in it which comes in a white bottle or as a blue jel. Basically these things are like a Datkin's solution. About 30 dollars a bottle, but less expensive than a vet.
An older dog may not be able to fight off foot fungus anymore, and all it takes is walking through one thing once and he'll be colonized. Thrush would make his feet feel like they are burning all the time, and licking would make it much worse.
Edit: Coconut oil may also help. It has some anti fungal properties as well.