My cat won't stop licking his cast!?

He has to have it on for at least another week, 2 if we can get him to tolerate it. But he won't stop licking it and the white stuff is pulling out and the vet wants me to put a cone on his head while I am sleeping and at work so he won't be able to lick it. I don't want to put him through the torture, he's already been through enough. Isn't there something I can put on the cast that will deter him from licking it? Thanks!