In short, There’s probably no legal way to tell without his cooperation or without having seized all his computers, phones and other devices and records through the use of a court order or other legal process (but consult a lawyer for legal advice).
The inventor of Bitcoin intended to protect the privacy of Bitcoin users and avoided having any data in the Bitcoin protocol that would identify people.
Bitcoin was designed to be used to hold money and spend it without needing any involvement by third parties at all, including no need for payment processors, exchanges or other businesses. There are no centralised records of who holds Bitcoin, it is like ordinary forms of cash in that respect.
Businesses that provide any kind of accounts for customers are typically required by law to protect the privacy of their customers. There are probably thousands of businesses that offer accounts for holding Bitcoin, many of these businesses will be in other countries than you are.
