A quick vet check may be a good idea… if anything, it'll reassure you it's not a health issue or allergy.
Do you provide something like a stuffed kong for him to lick at while he's crated? Stuffed FROZEN kong toys work wonders for crated dogs. the toy can be stuffed with tons of things (i.e. peanut butter, cream cheese, kibble, wet dog food, chicken, tuna, even jam a few dog biscuits in it...) it would give him something to focus on besides his back feet.
Another way to help (if it is an anxiety/nervous behavior) is Rescue Remedy (or another Bach remedy). these are natural extracts, that work over TIME to help ease anxieties.
