My partner sings along to music when we're in public. He starts out humming a few lines but then eventually goes into full-on singing as the music continues. It's embarrassing because he gets into it like he's in a recording studio. Yes, even does the hand on his ear when he’s reaching for a note. It's weird! How should I tell him?
Imitate him. Or record him and show him what he looks like.
If you really want to zing him, ask him which artist sings the song and when he answers, reply with, “Perfect, let him/her sing it.”
Does this only happen in public? Or does he bust out his American Idol audition around the house or in the car? Is he trying to get attention from people? Is he hoping people whip out their phones and start recording him? Is he trying to be a character and make people laugh? What is his purpose?
A couple of words come to mind here: communication and compromise. Tell him it’s embarrassing when you’re with him, especially if his expectation is getting attention from onlookers. Secondly, if you can tolerate his impromptu performances, tell him he can do it around you but not strangers in random public places. (Plus, it's just annoying. Have some consideration for people who don't want to be in a noisy environment. It's bad enough when people have their phone calls on speaker and we all have to hear it; or when people watch videos on their phone without headphones and we all end up listening to it.)
If need be, ask him straight up: "Have you ever heard someone singing in a (insert the public place here) before? No? Then it's not a place for singing."
If he has an interest in singing for strangers, go to a karaoke bar. There are plenty of people to entertain who can record him in all his glory. That’s the perfect time and place for his moment in the spotlight. And if he’s a terrible singer, well, hello, it’s a karaoke bar. That comes with the territory.