He sang an emotional track called "Used to You," which is off his This One's For You EP and you can watch the performance above.
Hours before the performance, Luke shared that he would be performing the song and shared his thoughts that "music is a healer." The full statement reads:
Tonight, the band and I were scheduled to tape a performance at @jimmykimmel Live! to air on TV later this fall. While we were on set today, Jimmy asked if I would be willing to perform a song tonight (11:35|10:35c on ABC) to honor last night’s fallen; I immediately agreed and we will be playing “Used to You.” Music is a healer. In this overwhelming darkness, I believe we will find hope when we rise together. Evil will not win. Fans, it is the blessing of my life to get to make music with you and for you every day. Tonight, may we find light and hope in the chaos.