The way you desensitize them to feet is by making ninety thousand positive touches and one negative one. So you use something he's really wild about, like cheese or cookies, and just start touching his feet. Sometimes you have to start by just putting your hand within a yard of his feet and moving closer. You associate foot touching with food or ball or whatever it is, and you really do have to think in terms of about ninety thousand repetitions. Once he's casual about doing it with food, play with his feet and tail constantly; make it part of petting. Teach high-five (Clue LOVES this--she'll high-five me a hundred times a day if I let her) or shake paw too. That way when you have to do something negative, like push his feet around looking for infections or clipping nails, the dog doesn't completely freak.
A training book I am currently looking at is very big on using what they call an intermediate reinforcer, which is a fancy way of saying that you don't ever sneak up on a dog and you don't wait until the dog performs to get him a treat. You announce your intentions and give the dog a clear view of your hand coming, and the treat comes into play as soon as you begin your movement. It's like asking for a napkin and saying "thank you thank you thank you" as the napkin is being handed to you, not waiting until it's actually in your hand. As a training method it's worth considering, but I think as an approach to introducing things that the dog will find unpleasant it's genius. I have started doing it for feet and tail with our puppy (who still doesn't like having his feet played with), where instead of what I would normally do--put him in my lap, hold his foot, clip nail, give cookie--I sit beside him and start him eating peanut butter or cheese immediately, I move in a way that he can see the whole time, and I repeat "good good good" in a very low, calm, happy tone as I am manipulating him. And then he gets a big bite when he's done. He's still not thrilled with it, but it's a calmer experience and he doesn't feel attacked.