So now people will cut off his hand to get his money!

Not the first time someone has injected RFID/NFC chips into their body.

Also, what happens when this chip dies? He won't have any early-warning signs. Or if he walks through a metal detector or something and interference causes the chip to go wonky, corrupting his bitcoin wallet.

To prevent this, he'll still need to have some alternative backup, like on a USB stick... which defeats the entire purpose of this experiment.