Kirath Mann suffers from Duchenne muscular dystrophy (DMD), which affects one in 3,500 boys in Britain and leaves many in a wheelchair before their 10th birthday.
Kirath's family, from Coventry in the West Midlands region of England, joined forces with other families to campaign to end delays to a breakthrough treatment called Translarna but failed due to high expenses involved.
"We are devastated at having to face yet more disappointment after another hurdle has been put in our way.
Kirath and five other boys with the condition even wrote personal letters to British Prime Minister David Cameron in June in a bid to get the drug that could save their lives.
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"Hello Prime Minister, my name is Kirath. I am six years old. My legs are poorly. Please help me get the medicine I need," read the letter in his childish scrawl.
"It was at NICE's discretion to give a positive decision and it was in their remit to do that. But they decided to go for a different approach," she added.
The drug - also known as Ataluren - is the first ever to tackle the causes of DMD and was approved in Europe in August last year and is available in France, Spain, Germany, Italy and Denmark, but not in the UK.
"It could potentially prolong the time before children have to use a wheelchair, compared with best supportive care," Prof Carole Longson, NICE health technology evaluation centre director, said.