The "Maggie May" rocker announced in 2019 he was "in the clear" after an intense course of radiotherapy for his own battle, with prostate cancer, two years earlier.
Tom, who was diagnosed with an inoperable stage 4 brain tumour in October 2020, was blown away when Stewart contacted him to offer his support.
Parker told The Mirror on Wednesday (08Sep21), "Rod messaged me saying, 'Listen, I've been through this situation. If you ever need to rant, I'm here for you whenever.' What a lovely thing to do, It was so nice of him to do that."
Members of the boy band, which formed in 2009 and broke up in 2017, announced on Wednesday they are reuniting for a concert at London's Royal Albert Hall on 12 November (21), with all proceeds going towards cancer research.
Bandmembers Max George, Tom, Nathan Sykes, Jay McGuiness, and Siva Kaneswaran had originally planned to reform for their 10th anniversary in 2020, but the pandemic forced them to put that plan on hold, until now.
The band also dropped a new single, its first in seven years, on Wednesday, and announced a greatest hits album is en route too.
Tom told The Sun, "It feels amazing, it is quite nice to be able to do something that is not related to treatment, it is like nothing has ever changed."
The band members shared a video announcement with fans, which they captioned on Instagram, "Here we go!! We are MEGA excited to announce that we are BACK and will be releasing our Greatest Hits Album on November 12th!!"
"We'll also be performing LIVE at the @royalalberthall as part of @tomparkerofficial 'Inside My Head' concert. It's been far too long... Let's do this!"