
Maria
You don't say how old he is, has it started later in life, or has he always done it?
Chewing a paw is often a sign of boredom, is he left for long periods of time?
If when the boot comes off and it is healed completely ie no broken skin, she could try bitter apple spray on it, they don't like the taste.
If the medication is working short term it sounds like it might be an irritation of some type, have they taken a skin scraping? Personally I would change my vet if he just shrugged his shoulders at me.
Perhaps someone else will come along with some other idea's
Sandra