I'm not ready for this to end.
I'm not ready for you to leave.
I'm not ready to say goodbye to someone like you again...
His Theme is the 90th song in the Undertale Soundtrack. It plays during the final part of the fight with Asriel Dreemurr, starting as soon as Asriel has a flashback of when he first found the First Human. It is a solemn, emotional song which is actually an orchestrated version of Memory.
It plays at 85% speed in-game. Part B loops in-game.