I wouldn't worry overly about your son. I agree with NY in that it is most likely he couldn't hear the words or was really even paying attention to them.
When I was 9 "Grease" was out -- I saw it at least 4 or 5 times at the drive-in, my friends and I singing out hearts out word-for-word to each and every song. Yep, that includes "Greased Lightning" ("you are supreme, the chicks'll #*$^ for greased lighting!") as well as "Sandra Dee" ("Look at me, I'm Sandra Dee, Lousy with virg.....") Oh and did I mention my name is Sandra so I REALLY took pride in beltin' out that one!

Regardless, I had NO idea what I was singing -- I just liked the beat and the music. My mother never worried about it and I was no worse for the wear. Though I must admit when I heard it again as an older teen - amusingly embarrassed, I said to my Mom "I can't BELIEVE you let me do THAT!"
Point being, I wouldn't stress myself out too much about it affecting your son. However, I must say I DO totally agree that with elderly and other folks sensitive to that sort of thing obviously in attendance, those sound like inappropriate choices.
Good Luck -- I certainly respect standing up for what you believe in!
