Born with just half a heart, little Ellis Holtom cannot run around with his friends like other three-year-olds.
The youngster was born with rare Hypoplastic Right Heart Syndrome, where the right side of the heart is underdeveloped.
The incurable condition means Ellis' heart is undersized and too weak to pump blood around his body.
He has already had four life-saving open heart surgeries and five other procedures and receives a lot of support from charity Little Hearts Matter.
On Sunday, his cousin Tamsin Carelse (pictured below) from Upfield, Shirley, will run the Paris Marathon in his honour.
The 26-year-old who ran the London Marathon in 2011, said: "I am running for Little Hearts Matter to raise awareness of their work.
"I don't have to fundraise because I have paid for my place in the race, but it is something I really want to do for Ellis.
"Ellis' condition means he doesn't have the stamina or oxygen levels as a child with a normal functioning heart. He has spent time in intensive care where it has been very touch and go.
"Ellis will always need special care and is never going to live a normal life.
"I asked his parents who they would like me to run for and they chose Little Hearts Matter, because they are so small and they don't get as much support as bigger charities."
Miss Carelse who has trained with the Croydon Striders, has already raised over £2,270 for the charity. She now hopes to meet her new target of £3,000.
To sponsor Tamsin visit virginmoneygiving.com/TamsinCarelse.