The 32-year-old singer broke the devastating news of his stage four glioblastoma diagnosis to Britain's OK! magazine on Monday (October 12) and George rushed to offer his support.
"I think the bravery of Tom and (wife) Kelsey announcing their situation is incredible," Max wrote in his post on Instagram. "It's obviously a devastating and scary time... but for those who don't know Tom I will say this. There is nothing that he has ever given up on and has never taken no for an answer."
"His heart is the size of a lions (sic), and it is his drive and passion that has always lead The Wanted and made sure we are looked after the right way. Speaking as a brother, a friend and a band mate... I know that you will conquer this as you have every other challenge you have ever faced. We are all on this journey together... and I can't wait to get back on the stage with you and the boys and have a proper celebration when you're better. You got this my man. I love you, Kels and all the family with all my heart and I'll be right here."
Breaking the news on Monday, Parker, who is dad to a 15-month-old with another baby on the way, said, "I'm still in complete shock. It's so much to take in... You never think this will happen to you."
While the average life expectancy for people with a stage four glioblastoma is between 12 to 18 months from the time of diagnosis, Tom has opted not to ask how long he's got left.