Maybe he doesn't necessarily have a specific meaning about the reason why you changed your avatar, but it's just an ordinary social topic, or maybe he has some personal interest or concern about your avatar change.
You can gently tell him why you changed your WeChat avatar, or share some related stories or ideas. If you don't want to share too much information, you can simply answer his questions or euphemistically say that you don't want to talk about this topic.
In addition, if this male colleague often pays attention to the changes of your WeChat avatar and asks why, it may also indicate that he is interested in or concerned about your social dynamics. If you think this will make you feel uncomfortable or uncomfortable, you can consider adjusting your social media usage habits or having a frank conversation with him to avoid unnecessary troubles or misunderstandings.