A Melbourne teenager has recorded an emotional video message to say goodbye after he was told by doctors he may not have long to live.

Shaun Wilson-Miller, 17, has battled heart problems since he was a child but last week he was given his worst diagnosis yet.

A biopsy revealed his body was rejecting his second heart transplant and doctors say they cannot perform a third.

Shaun, who became a Heart Kids Ambassador at the age of 13, filmed a video titled "My Final Goodbye" in which he thanks his family and friends and tells them he will miss them.

"This has been an awesome ride and I have no regrets," Shaun says.

"Live life to the fullest because you never know what's going to happen."

Shaun's father Cameron told ninemsn he was extremely proud of his son, who has become an inspiration to many others with health problems.

"He did that all himself. At first I didn't want him to make the video and I told him not to," Cameron said.

"But it's out there and it's touched a nerve all over the world."

Cameron said his son wanted to spread his message about staying positive to as many people as possible.

"He just wants to be positive in life, no matter what happens to you in life, be positive," Cameron said.

"He's just going to deal with the cards he’s been dealt and that’s it."

Shaun, who is writing a book about his life, has already inspired many people through work he has done as a motivational speaker.

"He's always just had that confidence to get up and talk even from a young age," Cameron said.

"He's helped lots of heart kids. He's even helped nurses with their relationships. He's like a guru. He just sits down and talks to people and changes their minds."

In 2010 Shaun struck up a friendship through Facebook with boxing champ Garth Wood, who that year defeated Anthony Mundine while Shaun sat ringside.

"Garth Wood says he's his inspiration, he was the motivation behind his fight against Mundine," Cameron said.

True to his message, Shaun also revealed positive news in his video — he has his first girlfriend, a 16-year-old who also battles heart problems.

"This girl has sat by his side and they inspire each other," Cameron said.