BRT- Dogs usually lick their butts and scoot when there is a problem-such as anal glands or worms. Paw/nail bed fixation tends to be allergies. Imagine an itch that you can’t scratch-kind of sucks, lol. I would make sure that he is worm free and his anal glands are not a problem. Tapeworm can not be seen on a fecal, so check around his butt very very carefully for anything that looks like hard dried up rice-those are the worm segments. You can try rinsing his feet to see if that helps-the other option, if the vet has already looked at him and said no issue-is to try a squirt of an anti chewing spray. If its merely behaviorally, he should stop after a few licks of bitter apple or something of that nature.
