The boy has been walking, running, cycling and swimming to raise money to support his brother, Reuben, who is suffering from an unnamed, rare condition. Over the course of 3 months, Harry Byrne, 7, has been trekking more than 13,500 miles for his little brother Reuben.

       The condition has seen little Reuben’s brain deteriorating over time – but doctors are unable to name the condition, due to the rarity of it.

       Across the world, there are fewer than 20 cases known to be active.

       Young Harry, from Larkhill, Wilts, was able to clock up an impressive 28 miles in a single day, as he and his friend walked from Avebury to Stonehenge – a journey which took them 12 hours to do.

       Sophie Byrne, 29, said she is “so proud” of everything that Harry has achieved through his walking challenge.

       Ms Byrne, who is also mum to eight-month-old Felix, said: “We’re so proud of Harry for everything he’s done.

       “Not only has he raised money and awareness of Reuben’s condition, but he’s also created all these amazing memories with his little brother, which is just priceless.

       “Harry’s favourite parts of the challenge were the bits where Reuben was there with him - like when he cycled the last mile with Reuben, or when I got into the pool with Reuben to watch Harry swim a mile.

       “We’re not sure how much Reuben can actually see, and he doesn’t really understand a lot of what’s going on around him.

       “But he does occasionally give a big smile or even a laugh, and that’s when you know he’s really happy, overjoyed - and I think those moments have spurred Harry on a lot in the last few months.”

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       But the 7-year-old could not hide his enjoyment of the whole experience.

       He said: “My favourite part was when Reuben and I went on the bike together. I really enjoyed that.

       “Some of it was really tiring, but having lots of support from friends made it easier.

       “I really want to keep raising more money for Reuben, because it makes me feel really good.”

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       Ms Byrne continued: “As far as doctors know, there have been fewer than 20 recorded cases of this condition worldwide - and two of them are my own children.

       “The longest surviving child with this condition lived to the age of just five years old.”

       The couple had a daughter Olivia nine years ago, however their daughter was born with the same unnamed condition, which she later passed away from aged nine months old.

       The 29-year-old added: “We’ve always talked quite openly and positively with Harry about Reuben, and about Olivia, too.”

       “It’s something that you can either let take you to a really dark place, or you can turn it into something positive - like Harry has done.

       “Things that happen in your childhood can really affect you in adulthood - so we wanted Harry to be able to look back and think of all the amazing memories he created with Reuben.

       “After Olivia passed away, Sean and I did the Three Peaks Challenge to raise money for the hospices that had cared for her.

       “And Harry wanted to do something similar to support the hospices that help his little brother.”