Ok, so I will presume the amount is in the millions.

The risk of the money being lost in fire/flood/etc is a concern. Simple theft is also a concern as burglary is not that uncommon.

At the end of the day, it is his money. He can make a fire with it if he wants. I would just present your concerns and at the end of the day, he will have to make the final decision. What else can you really do? He can also consider buying something with value (like property, precious metals, etc...) Maybe he just wants it in something he can touch?