I am not sure if you solved your problem, but I have two rotties and they display this 'licking' when they are bored. Change up his walk, bring him to new spots, be sure he has other dogs to play with, buy him some squeaky toys or tennis balls.My dogs love racquet balls.I am certainly not a vet, but normally licking is a sign of boredom. To rule out injury, I would feel around on his paws, squeeze a little bit and if he has no reaction to you doing this he should be fine physically. Just like humans, doggies get bored to! I hope this helps.