Jack, 6, was diagnosed with an inoperable brain tumour two months ago and discovered the benefits of music therapy while undergoing radiation treatment at The Children's Hospital, Randwick.

His parents Paul and Karyn Woodhams watched glimpses of their cheeky, music-loving son return with the help of the hospital's music therapist Matt Ralph.

Jack's experience of using the American rock band's tunes to help cope with his illness made its way to Simmons through social media and a global network of devoted Kiss fans this week.

The band and Simmons posted the story on their Facebook and Twitter accounts to an audience of millions with the message "Be strong, Jack!". Fans have also posted messages of encouragement and support.

Speaking from his Beverly Hills home on Thursday, Simmons said he'd been touched by Jack's story and was heartened to see the band's music used for a greater purpose.

"The initial reaction is, of course, how sad," Simmons said.

"The injustice of a six-year-old boy getting this debilitating disease is beyond words."

Simmons had watched a video clip of Jack's favourite song, Rock and Roll all Nite, which starred Jack, his family and medical staff and was organised through the hospital's music video program.

"We're not doctors but it's fantastic if Rock and Roll all Nite, which I'm proud to say I co-wrote, lifts his spirits and puts a smile on his face.

"It looks like his entire family is having so much fun.

"Before you know it, Jack will be signing autographs."

Simmons said Americans had created an image of a superhero that revolved around a strong, healthy man who stood on the edge of a tall building with a cape billowing behind him.

But Jack, he said, was the "real deal".

"That's my kind of superhero."

Simmons said Jack's story had highlighted the benefits of music therapy to a global audience and made thousands of people count their blessings.

"Jack is inspiring a lot of people to wake up and say, 'there but for the grace of God go I'."

The Woodhams family will be among the crowd when Kiss performs at Sydney's AllPhones Arena as part of their tour down under in October.

Mr Woodhams said Jack was "pretty stoked" about his newfound fame.

But he said his family didn't share Jack's story to bring attention to their son; they instead wanted to shine a light on the work of the children's hospital and its music therapy team.

"It's just good to see him happy, music makes him happy," Mr Woodhams said.

"He's always loved music, I guess that's why he responded so well to Matt in the first place."