Tell him it is a very good question and you will ensure this subject is fully covered in your training course.
You would then expect to react according to your company procedures training to cover this event
Say presently you are not a copilot and have not been trained to deal with this situation but if you insist I give you an answer I would not let him kill me, the passengers, or crash the aircraft.
I would act positively and reasonably and ensure it all sounded logical in the CVR. I would be happy to be alive without a job than dead and everyone knowing I sat there and let him kill us all.