Alexander Liggins, from Tiptree in Essex, was born 16 weeks premature while his parents were on holiday in Mallorca.
He has a range of disabilities and requires extensive care.
Family friends James Marshall and David Cross, from Harold Wood, have decided to tackle the Thames Path Challenge to raise money for Alexander.
James, from Brentwood, has been friends with Alexander’s family for 25 years and was in Mallorca when Alexander was born.
He said: “I’ve known Alex from when he was just a few days old.
“David and I are delighted to be raising funds and awareness for him.”
Alexander’s mother, Alison, said: “It was quite a shock when Alex was born; we’d only arrived in Mallorca six hours earlier.
“We were eventually repatriated back to the UK after 9.5 weeks.
“Alexander stayed in hospital for another 6.5 weeks, and then we finally came home.”
However, at nine months old, Alexander developed infantile spasms, a rare form of epilepsy that causes developmental delays.
Alison said: “We stayed in hospital for a month while we tried to gain control, and we did have a brief spell in remission, only for Alex to relapse again a couple of months later.”
“Alexander is non-mobile, non-verbal and PEG fed (through a tube).
“He was having up to 300 spasms per day at one point but this is now under better control.
“The main problem at the moment is dystonia, which is restricting the little movement he has as his left hip has become dislocated due to his muscles pulling it out.”
Despite his health issues, Alexander is described as a “happy little boy” who loves being outside and enjoys hydrotherapy.
Friends and family have already rallied around raising funds through events including a 35-mile walk and a mud run.
The family is focusing on raising funds for an off-road wheelchair and therapies to ease Alexander’s dystonia.
The Thames Path Challenge, which more than 2,000 people will take part in, will see participants walk from Putney Bridge to Henley over two days.
Alison said: “It is a really difficult challenge and Matt and I are both grateful beyond words that James and David have chosen to do this for Alexander.
“We wish them the very best of luck.
“Matt will be there at the end to give them a well-deserved hug and to buy them a beer.”
The family is being supported by children’s charity Tree of Hope, which helps families raise money for medical treatments and healthcare services not freely available through the NHS and social care.
Becky Andrew, Tree of Hope’s chief executive, said: “We wish Alexander and his family all the best with their fundraising activities and best of luck to James and David for the Thames Path Challenge.
“We are pleased to give them support in reaching their fundraising goals.”
Anyone wishing to donate to Alexander’s fund or to sponsor James and David for the challenge can do so on the Tree of Hope website or on JustGiving.
