When he’s not crowd-surfing on a giant inflatable animal, Andrew McMahon is raising a million dollars for young adult cancer patients and survivors.

Andrew McMahon of Andrew McMahon In The Wilderness greets the audience while performing at the American Express Stage during weekend two of the Austin City Limits Music Festival at Zilker Park in Austin, Texas, on Friday, Oct. 13, 2017. TINA PHAN / FOR AMERICAN-STATESMAN

He started his charity project, the Dear Jack Foundation, after being diagnosed with — and surviving — leukemia in his early 20s, and in the State Farm “Here To Help” lounge at Austin City Limits Music Festival on Friday, he explained why: he saw a “chasm” in services for young adults who had survived or were battling cancer, and he knew from experience that there weren’t enough resources.

“I was an unwitting guinea pig,” he said.

During the private evening performance, McMahon and bandmate Zac Clark played a stripped-down show consisting of just three songs (that he hadn’t played in his set on the Honda stage earlier in the day) before talking about the importance of giving back to their community.

Getting ready for a private performance with @andrewmcmahon in the State Farm #heretohelp lounge at #aclfestpic.twitter.com/oLdRPv586b

— Katey Psencik (@psencikk) October 13, 2017

An all-keys version of “The Mixed Tape.” @andrewmcmahon#aclfestpic.twitter.com/a2BPPrIA2m

— Katey Psencik (@psencikk) October 13, 2017

“That was one of the hottest sets I’ve been onstage to play — I think I just woke up from the blackout,” McMahon said before playing “High Dive,” then laughing his way through an explanation of some of the lyrics in an early Jack’s Mannequin hit, “The Mixed Tape.”

“There’s a line in the song that says she broke into my house,” he said, referring to his then-ex-girlfriend, now-wife, “And she’s like, ‘I did not break into your house, I had a key.’ …she did have a key, but at the time she was not invited.”

Before playing “Fire Escape,” he pointed to his wife’s cousins in the back of the room and held a finger up to his lips, then told a story about hazy nights in New York City that inspired his final song.

Also in the audience was Katie, McMahon’s sister who famously saved his life when she was a perfect bone marrow match during his cancer treatment. He used his story to encourage others to volunteer in their communities and find what matters to them.

“I was affected by something so profoundly that it made me want to figure out how to participate,” McMahon said. “It’s a crazy thing but to give things away is so much more of a gift to yourself.”