His estate owns it, so only the executor or the administrator of his estate can deal with it during the probate process.
As a practical matter, most financial institutions immediately freeze the accounts of deceased individuals when they learn of their deaths.
The deceased person no longer owns anything for you to handle for him because he can't legally hold money or property.
The POA might authorize you to make financial transactions for him, but he technically no longer owns the property or the money over which the POA placed you in charge.
You're the individual he wanted to take care of certain personal business matters for him.