My dog licks his foot non-stop, for no reason.?

We've taken our dog (3 year old cattle/corgi mix) to the vet at least 3 times. There is nothing wrong with it, as far as the different vets can see. We've tried spraying this on it there are designed to deter dogs from licking, deodorant, socks and now he has had an e-collar on it for 2 weeks and the Doctor gave us steroid spray to put on it twice a day. But, I noticed that he was manovering his cone so he could still lick it. The doctor thinks it is an OCD disorder. Please help our little cute doggy.