Oliver Brown has been diagnosed with Myelodysplastic Syndrome – a condition with just three other known cases in the UK.
It was only discovered after routine tests revealed Oliver had a dangerously low blood count.
Doctors are now in a race against time to find out more about the illness while his parents after trying to raise enough money to ensure Oliver can undergo a second bone marrow transplant if the first fails.
The family are trying to raise £120,000 crowfunding via JustGiving to pay for what could be a life-saving transplant for Oliver – but are still searching for a match.
The condition is a complicated genetic condition relating to his GATA2 gene, which doctors believe may be why Oliver lost his hearing.
But his parents, from Plymouth, Devon, said strangely after the diagnosis things started to make sense.
“Oliver has always been special,” dad Mike, 37, said.
“When he was younger we used to think he wasn’t brushing his teeth properly as his gums used to bleed.”
Oliver has also been deaf since birth – which doctors now believe is related to this rare condition.
Mum Nicola, who works at a dental school, said: “The most difficult thing is knowing there wasn’t anything I could physically do to make it better and that we are very much in the hands of the medical team.”
“This genetic link was only discovered in the last five years, so the process of collecting cases is still going on. As yet, we don’t have a population-scale analysis to know how common it is.”
The head of research at a leading blood cancer charity said Oliver’s condition was extremely rare and said it had only been discovered around five years ago.
Matt Kaiser, of Bloodwise, said: “This genetic link was only discovered in the last five years, so the process of collecting cases is still going on.”
He said: “There have been reported cases of people inheriting a faulty GATA2 gene before, but it is incredibly rare – fewer than one in a million people have the fault but if a person does, then they have a 50:50 chance of passing it on to their children.”
The family have been wrapped up in a whirlwind of hospitals, blood tests and treatments since Oliver was diagnosed last summer.
He was rushed to hospital in the middle of the night after doctors thought he might have lukeamia.
Nicola and Mike, an engineer for the Ministry of Defence, became concerned after Oliver developed a cough on holiday.
He was taken to see a GP who conducted blood tests. When the rests came in that evening, the couple had a call from the lab who called him into hospital immediately.
A few days later, they – and little Oliver – were presented with a team of doctors who told them the news that his condition was far rarer than they had expected.
“Initially it was overwhelming, I think as a parent you get on with on and at the minute we are just trying to tackle challenges but the hardest thing is not knowing what the future holds.
“At the moment nobody can tell us what will happen, it’s scary and that is a very difficult thing to accept.”
While the future remains uncertain, Nicola said the situation has changed the way she sees life.
“It has changed my outlook, you cherish the time you have together. Christmas was particularly special this year and whenever Oliver is well enough, we are making the effort to go out and make the most of these opportunities.”
And throughout it all Oliver has remained remarkably brave.
“Even when he is being wheeled into an operating theatre he is making jokes with the nurses and hospital staff,” Mike said.
“Throughout his life he has just taken everything in his stride and I’m so incredibly proud of him,” Nicola added.
Oliver made his younger brother Benjamin, 5, a special keepsake to comfort him while he was away.
Nicola explained: “With Oliver in isolation, Benjamin can’t be in the same room as him which is really upsetting as they are very close and he misses him a lot.
“So Oliver went to Build-a-Bear before he went back into hospital and recorded a message in a teddy for Benjamin so he wouldn’t miss him so much.”
The family try to Skype and FaceTime as often as possible to keep things normal for the children.
Mike said they have been overwhelmed by the support.
Godfather Rob Tremaine managed to raise £3,000 for shaving his head in solidarity with Oliver, whose hair was cut this week.
“It’s not just about giving money, people can help in all sorts of different ways,” Mike explained.
“Giving blood is so important and it really can save someone’s life, Oliver has had so many blood transfusions.
“People say what can I do to help or is there anything I can do, but usually by that point it’s too late.
“If people want to help, giving blood or donating bone marrow can be lifesaving.”
The couple also praised the staff at Bristol Children’s Hospital and Derriford in Plymouth and said they had been overwhelmed with support from their friends and family.