How sad if he had lost it. Somehow if there is anyway you can just keep your keys at your house encrypted in such a way that only you understands it would be better. Or how about spreading the keys across walls of your house or bedroom written very small? Maybe it will help. If saved in a computer you might have problems keeping it due to possible hackers. I think there are so many ways to back it up manually those are just my suggestions though to avoid such problem.