Kayden Hurrell suffers from ornithine transcarbamylase deficiency (OTCD), a condition that causes dangerous levels of ammonia to build up in the bloodstream.
The schoolboy from Greenock, Inverclyde, was diagnosed last year and has to take medication and eat an extremely healthy diet to keep his symptoms at bay.
His mum Paula, 26, says the condition runs on her dad’s side of the family, with both her and her sister Tina, 19, suffering from OTCD.
Paul’s 11-month-old son Joshua also has the condition, although middle son Ryan, who is almost two, doesn’t.
If left untreated the disorder can be deadly.
The toxic compound is a waste product which forms when the body breaks down protein.
But a lack of OTC enzyme needed to flush it out creates an accumulation and can lead to severe headaches, sickness, loss of appetite, tiredness, and confusion.
In extreme cases, the hereditary condition can cause coma, brain damage, and even death.
Mum Paula said: “Kayden mostly suffers from sore heads - the way he describes it is a ‘sore brain’.
“Even a bump on the head at school could affect his OCT and he has special sugary drinks that he gets which help keep his levels right.
“It’s really worrying as a mum, especially because Kayden is quite a clumsy wee boy.
“And for Joshua, when he is ill and we take him to the doctors, he is always taken straight to the hospital, it can be terrifying.”
Paula was diagnosed at the same time as Kayden last year but said she didn’t know a lot about the disorder at the time.
She said: “It runs on my dad’s side of the family, so my sister and I both have it.
“A lot of my family were being tested for it last year so I went and got Kayden done at the same time and we both had.
“At the time I didn’t know much about it at all.”
But courageous Kayden, a pupil at St Patrick’s Primary pupil in Inverclyde, doesn’t let it hold him back.
Along with aunt Tina and grandmother Christina Hurrell, he is preparing for a 3.5-mile walk to raise awareness of the condition.
The walk will also be in memory of Paula’s late father Harry, who died in 2011 aged 47 -- he was a carrier of the OTCD gene and also had multiple sclerosis (MS).
Gran Christina, 51, from Port Glasgow, said: “Kayden’s body doesn’t break down protein.
“The side effects are a sore head, hallucination, sickness, and if it’s really bad he can go into a coma.
“I think what he is doing is brilliant.”
Kayden added: said: “I love walking and running - I’m fastest in the whole class.”
His aunt Tina, 19, who is organising the walk and is also a carrier said: “He has special medication because if he gets a sore head he can go into a coma.
“The walk is also in memory of his grandad, my dad.
“All the money raised will be halved between the MS society and the OTC department at the Royal Hospital for Children.”