To make sure he doesn't bite you when you are opening his mouth, do as others suggested, squeeze in at the hinges of this mouth and make sure his skin is on the inside. I've done this with dogs and cats- it works wonders! It was the only way I could get my cat to open his mouth for medicine he was on.
As far as making him drop it, I would suggest trading a treat for it. If he won't trade it, the treat isn't good enough.
