Duchenne is not so well understood as other serious illnesses, but it is just that - in fact, it's fatal. Most boys diagnosed with the muscle-wasting condition have a maximum life expectancy of 25 - up from 18 in the past five years thanks to advances in technology.
Coleman presented so severely early on pediatricians feared he would not make it to double digits, but they didn't bank on his mother's dogged determination.
"He's definitely going to make it to double digits" Ms Stephenson says.
She puts his progress down to a group effort by friends, family and teachers at Kowhai Special School who keep him moving, comfortable and learning every day.
Pediatricians Kai Steinmann, Kate Robertshaw and the staff at the Hawke's Bay Hospital also played a large part, as did CCS disability action, Muscular Dystrophy Association and Options Hawke's Bay.
It affects just one in 3600 boys and is caused by a mutation in the dystrophin gene on the X chromosome - usually passed down from mother to son, or in Coleman's case a completely unexpected mutation upon conception.
Devastating news came for Ms Stephenson when her son was just three and not developing at the rate of his peers.
A visit to the pediatrician delivered a preliminary diagnosis - blood test was confirmation - from there a muscle biopsy. Then the journey to help him walk by age four, with plenty of persistence and physiotherapy.
Now he is one of only three New Zealand boys participating in a PTC Pharmaceuticals Translarna trial in Australia, a promising potential drug therapy.
Costs associated with travel had a big impact on the mum-of-three's budget. But progress since he began taking the drug a year ago was visible.
Coleman has gone from sleeping two to four hours a night to settling for longer periods, though Ms Stephenson is always on guard. "I am constantly checking up on him."
If she were to fund the treatment herself it would cost about $300,000 per year, but there are plenty of added costs as it is, including other medications and top-ups of nappies.
Currently she is trying to come up with $3500 for a new wheelchair so Coleman can go offroad to the beach and feel sand between his toes.
She also has two other children, a 7-year-old boy with a developmental delay who had a seizure for the first time this week and will require a scan, as well as a 5-year-old boy.
Boys with duchenne often have enlarged calf muscles, difficulty walking, frequent falls, fatigue, muscle fibre and skeletal deformities, among other things and will usually be bound to a wheelchair by the age of 12. They are eventually left paralysed from the neck down and require 24/7 care.
Coleman recently became the face of Muscular Dystrophy Association of New Zealand's latest fundraising and awareness campaign, which runs throughout September and October.
Ms Stephenson started a Facebook page called Coleman's Crusade to share his progress with friends and family. It has heartfelt posts, photos and updates (facebook.com/colemanscrusade).
-If Hawke's Bay people would like to support Coleman financially they can make a donation on his Givealittle page: Coleman's Crusade.
