How to communicate to him that he needs to keep out of things that don't concern him?
I would definitely talk to him outside of the workplace. And tell him a few things so he hopefully understands why, while at work, I want our relationship to stay on professional tracks. It will deflect the personal grievance to a professional ground (where it actually belongs).
- While at work, I am a paid professional who want to work hard and be rewarded for it.
- I don't want other employees to think that I'm favored by a friend. You're my manager, not more, and should be seen just as that by our co-workers (and it makes me feel uncomfortable they could see it differently).
- I'm afraid that any raise or reward I would get could be seen by colleagues as unjustified because of our friendship. This could be a barrier to my career.
In some places/countries (i.e. US), company policy can even take this as conflict of interest (see Workplace SE for a lot of info about that: managers, friendship at work and so on...).
Then, explain how you still want to be friends, and be happy to talk to him, but not at the workplace. Even though being friends and colleagues can lead to weird situations, the best way to dodge them is to separate work from home/friends.
Then, depending on his reaction and future behavior, you'll have to adapt...
As someone who has navigated workplaces in different countries and with fellow co-workers from different countries and cultural backgounds, keeping personal doors closed while at work actually never bothered anyone. And many did the same. Be professional before everything else. Your friend will have to understand that. Choose your words accordingly, and the proper time.
1. workplace.SE - manager and close friend.
2. Forbes - new rules
3. Manager resource - manage a friend
4. Huff. Post - office friendship