With the latest movie offering the biggest hint yet at their connection, Ian McShane responds to the Winston family fan theory in John Wick: Chapter 4. McShane's character has been a key ally for Keanu Reeves' titular assassin throughout the franchise, offering him support via the New York branch of The Continental, even at the risk of angering the High Table. Chapter 4 further pushed their connections as Winston was shown to have knowledge of John's Russian heritage, and even expressed "Farewell, my son" in Russian upon burying John at the end of the movie.

https://screenrant.com/john-wick-4-winston-father-son-fan-theory-ian-mcshane-response/