Daniel Greer from Newry is currently in the Royal Hospital for Sick Children in Belfast after receiving the devastating diagnosis earlier this month.
Doctors have told the boy he will require a bone marrow transplant which may save his life and his family have now started campaigning to get people across Northern Ireland to sign up to the donation register.
Despite this, doctors have informed Daniel’s family such a match may be as rare as one in 10 million.
On Facebook, Daniel’s older brother James explained the blood cancer has spread “rapidly and aggressively”.
“He is in hospital and has just completed 10 back-to-back days of very intensive chemotherapy, sometimes having to take four doses in a single day,” James wrote.
“During this time, he has needed three units of blood and two platelet transfusions.
“If you know our Daniel, and most of you do, you'll know he never backs down or gives up easily. He is battling through it like the little fighter he has always been.”
Speaking to BBC Talkback, mum Anne said the family want to spread the message out “far and wide” in order to encourage people to sign up for the register.
“We are a small family Daniel has one sibling so there is only a one in four chance his brother will be a match. I am not willing to sit back and wait,” she said.
“To be told your child has a potentially fatal illness and you are looking at a well boy who climbs the Mourne mountains and is an avid sports person.
“It is like waking up in a nightmare, would be the best description I could make of it.”
Anne explained they were alerted to the problem after Daniel had initially complained of neck and back pain
“Two very specific points, one just at the very back of his head and one just on the bone at his waist level,” she added.
“Two weeks previously he had been scuba diving with my husband and we thought it was the tank on his back.
“It was the Saturday morning when he got up, he just looked pale, drawn and he looked sore.
“All I could say is I would appeal for anyone who is eligible to sign up to the bone marrow registry.
“Anybody who is listening to this who is thinking this is terrible. If you were put in the position we are finding ourselves in, if you would take a bone marrow or stem cell donation from a complete stranger - you should be on this register.
“I hope this message goes out far and wide.”
The family have encouraged people across Northern Ireland and the world to go online to the DKMS website to register as a donor, with the registration and subsequent swab test only taking a matter of minutes.
“He is trying his best to keep a positive face on to keep everybody laughing. He has become something of a celebrity in the sick kid's ward,” she said.
“He has a wicked sense of humour but this last couple of days he hasn’t been as well with a high temperature. It is very hard as a parent to sit and watch your child going through that and feeling your hands are tied.”
To register as a potential donor, go to www.dkms.org.uk to sign up.