A friend of the Bee Gees singer told the Daily Mirror newspaper: "He's very strong willed - if anyone can beat this it's Robin."
Despite feeling "very unwell", Robin was upset at missing out on a charity event for Rock The House - which was started by Conservative MP Mike Weatherley to support musicians from across the UK.
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Speaking at the event, his spokesman and long-standing friend Mick Garbutt explained: "He would have absolutely loved to be here in person, but unfortunately that simply hasn't been possible with his health.
"It is true to say he is very unwell, but he's an incredible man and enjoys lots of support from his close circle of friends and family, and particularly from his wife Dwina.
"He's determined to keep supporting good causes which he believes in strongly even as he fights this terrible illness. 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."
Despite his commitment to the charity, Mick admitted Robin is "very unwell".
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."
Robin - who lives in Thame, Oxfordshire, South East England with Dwina - is determined to beat the disease and plans to attend the launch of his new classical composition in early April.
Mick said: "He has set himself a goal to be at the gala performance of his first classical composition in April and everybody around him hopes that he will be there."
In November 2011, Robin - who is a teetotal vegan - revealed he was suffering from liver cancer, which was diagnosed in April.