Robin Gibb is undergoing chemotherapy in a bid to beat liver cancer.
The Bee Gees star, 62, was admitted to hospital in November and cancelled a number of high profile performances after he was described as "frail".
Mick said: "He's very strong willed - if anyone can beat this it's Robin."
He added: "He is receiving regular chemotherapy and fighting hard. He is also heading home to be with his family at every possible opportunity as that's where he feels comfortable, but the reality is that he is very unwell.
"He's an incredible man and enjoys lots of support from his close circle of friends and family, and particularly from his wife Dwina."
The musician, who is resting at his £8m home in Oxfordshire, was apparently devastated to be missing a charity event for the music project Rock The House but he is determined to still support the causes that are close to his heart.
"He's determined to keep supporting good causes which he believes in strongly even as he fights this terrible illness," said Mick.
"The support he has been receiving from fans has been fantastic - he's a national treasure really. While he can still use his profile to raise awareness for issues close to his heart he will always do so."
In a statement posted on his website, Robin said: "I wish to thank my family, friends and fans and the many thousands of people who do not know me but have enjoyed Bee Gees music and have wished me well."
He added: "I have been very unwell and am now on the road to recovery."