you could probably just tell them it bothers you, if you do it in a chummy way. i've had two jobs where i'd sing constantly -from rammstein to beethoven's ninth. i expected to be asked occasionally to stop, and always acquiesced; everyone seemed okay with the compromise. i'm still friends with three of my coworkers and stayed a summer with my old boss and his wife.
[at my most recent job i had my own lab so i was only heard when the labour-people were in the break room for lunch; i used to stop during these periods because i didn't want to annoy them, but they'd knock on my door to ask why i'd stopped.
only, when i actually played beethoven's ninth they said it sounded like dogs barking; so i don't exactly take their appreciation as a compliment.]
anyway, if he's singing, i'm sure it has occurred to him that some people won't like it; he's certainly in no socially-strategic position to act like you're being obnoxious for wanting some quiet. you have the advantage in being the one NOT intruding on other's pursuit of happiness... or productivity, at least.
