I don't think it's a matter of him being a bad singer, he certainly is mediocre at best, but I think he wasn't used to the key change, and some people (who aren't very musically gifted) have a bad sense of pitch.
The original song is in c minor, but they usually perform it live in b minor (one half step lower). However, this acoustic set was even LOWER (in a minor) and Louis was singing as if he was singing the song in b minor.....
Edit: He definitely wasn't sick. It was his bad sense of pitch and key that made him flop
