“When did he go through the naturalization process?”
The right to acquire naturalized U.S. citizenship vested at birth and is perfected after birth by compliance with the statutory requirements. This is described in the U.S. Immigration and Naturalization Act of 1952 and administered in the U.S. State Department Foreign Affairs Manual.
How about- when he declared that he was giving up his Canadian citizenship- for $200 Alex?
Overview
This application is for an adult Canadian citizen who wishes to renounce his or her Canadian citizenship. To renounce your Canadian citizenship you must prove you are a Canadian citizen. You must also prove that you are, or will become a citizen of a country other than Canada, if this application to renounce is approved.
http://www.cic.gc.ca/english/citizenship/renounce-how.asp