It sounds rather quaint and old-fashioned. Like something a teacher might have said to a student or an army captain in the 1950s. I think of the movies my grandfather used to watch on Sunday afternoon, all of which were set around the time of the second world war. Also characters like Mr Banks from Mary Poppins! <https://www.youtube.com/watch?v=LZXITCwBdJQ> Other expressions in a similar vain include: My dear boy Yes old boy! My dear fellow! Of course, old chum! It depends on whether your intention is to sound like an old English gentleman Jean! (You might do this to make your friend laugh, by saying 'my boy' in an accent like the one from the video)

Terms of address like "my boy, my guy, my man" can have a lot of different connotations. None quite seem to fit to what you were looking for, though ;-). (1) My boy - Like Rob said, the first thing that came to mind was an elderly gentleman addressing somebody junior, definitely 1950s or earlier: "Now, listen 'ere, my boy, that's not the way we like to do things around 'ere ..." If you leave out the "my" it gets a very racist undertone. (2) My boy, part 2 - "Now, that's my boy!" - a proud father seeing his son strike the ball out of the park ... (3) Similar: My guy ... as in the song (nothing you can say can tear me away from my guy). Note that this would definitely mean "boyfriend"; also possibly a bit dated (4) Similar: My man ... also for a (serious) romantic relationship, if used between man and woman; between men: best buddies - yo, that's my man! Both may be more appropriate in some cultural settings than others. Conclusion: all of these expressions have a lot of connotations which can only be learned from exposure; I would stick to more neutral terms ;-)

That man is a rude