Leave that to him.. If he likes you, he should be the one to gather courage and tell you.
No matter what the circumstances are, if he likes you that much, he will tell you.
I would suggest being direct. Most guys are not good at guessing deeper meanings. However, if you know he likes to solve puzzles, then write "Look beyond the lyrics" or perhaps "Can you find the secret message?" on what you give him. Also, I would suggest giving it to him in person. If you are afraid to do that, then be sure to write your name on what you give him. The idea is cute; however not everyone would be able to understand its meaning. Something else to think about is whether him being unable to decrypt your message would make you think less of him.
Or you could ask him if he finds you attractive?