and there’s nothing there that he’s licking? if not, it’s often boredom. give him a kong with peanut butter or play fetch for a good 15-30 minutes
This is one of the biggest things people don’t see. Boredom. This is compulsive behavior (if food is ruled out). I bet that when the dog goes for the foot, he gets attention (interaction with his human-which is what dogs need). Exercise, exercise-both physically and mentally.
My sons FIL who trains dogs thought the same. I walk him 40 minutes a day regardless of weather but may need to increase his activity.