A Sydney family is moving to the US in an attempt to save their son's life.

When Harry Stilwell started kindergarten in Sydney last year, his teacher reported a problem. He had to get very close to his work to be able to see it.

His parents, Susan and Shawn, took their son to a string of eye doctors to find out what the problem was.

“We discovered that Harry was visually impaired to the point where he was declared ‘legally blind’,” Shawn explains on the family’s Event Brite page. “The deterioration was frightening and puzzling.”

Eventually, a neurologist delivered the devastating diagnosis: juvenile Batten disease.

‘He said, ‘He’s going to be sick and pass away,'” Susan tells Daily Mail Australia.

“I fell to my knees when they told us.”

Batten disease is a rare disorder. The signs are subtle at first, but as children get older, they suffer mental impairment, worsening seizures, and progressive loss of sight and motor skills. They become totally disabled, and then die, often in their teens.

“Unfortunately, there is no help for Harry in Australia,” Shawn writes. “Harry’s only hope is in the USA where gene replacement therapies are developing in clinical trials.”

The Stilwells are planning a move to the US early this year, with the hope of joining a clinical trial. They already have an appointment with a neurologist involved with the trials.

The family is holding a fundraiser next month to raise money for the Australian chapter of the Batten Disease Support and Research Association, and also for Harry's journey.

Shawn says Harry "really enjoys life".

"He has a wonderful, warm personality and is very cuddly."

Batten disease is inherited. If both parents are carriers, that means each of their children has a 25 per cent chance of developing it. The Stilwells have two other children, Oliver and Macy.

"We're just trying to put one foot in front of the other at the moment," Susan tells Daily Mail Australia. "I couldn't bear to hear."