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 Dreemurr 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 will constantly loop in-game.