If your best friend feels anything for you, then he's NOT going to hate it. Besides, you're both singers.
If you cannot find a suitable song among the many crush-related songs, then take a look at one of the two following Spinditty playlists:
1) Consider tunes listed under "Songs About Needing Someone" http://hub.me/al2lh. I specifically like "When I Need You" originally by Leo Sayer (1977) and "Baby I Need Your Lovin'" by Four Tops (1964),
2) Particularly if there are sparks between you, why not pick a kissing-related song? There's a long list of Songs About Kisses and Kissing here: http://hub.me/akFW8. I particularly like "Are You Gonna Kiss Me Or Not" by Thompson Square (2010) and "Shut Up and Kiss Me" by Mary Chapin Carpenter (1994).
Good luck serenading him! You've got this!