Can my nephew drive my fully comp insurance car, he has fully comprehensive policy as well?

Your nephew needs to check his insurance policy documents to see if there's a clause which says that he can drive another car with owner's permission. Not all comprehensive policies allow you to do that, so it's best if he confirms with his own insurer first. I'd probably give them a call just to be on the safe side.

Mind you that when your nephew drives your car (if he's allowed to do so), he will be insured third party only although he's got fully comp insurance.