Dogs can also develop body tics or compulsive disorders or just plain ol’ run of the mill habits. Your pup’s nail biting might be completely harmless and your vet seems relaxed about it. Something that your dog might turn to when he is bored or nervous. Or it could just become a habit for no other reason other than repetition
There could be many reasons, sometimes biting and licking of feet is indicative of pain higher up the leg, or it could be, as already suggested, something he does when he is worried as a calming behaviour or a habit. If he is not causing any harm to himself doing it and it doesn’t bother you then just ignore him. If you want him to stop then you need to start distracting him and maybe swap his feet for something he can chew!
Did your vet consider food allergies? Often times excessive biting or licking are symptoms of this. You might want to change his food and see if it helps. If you feed the same food, then try switching to something with a different meat source and/or starch. Read the labels and look for items with the fewest quality ingredients.
You will need to wait 2-3 months to see if there was any change.