Kenny Handford had contacted one of the world’s foremost experts on Leo’s condition and had been sent a trial drug that he hoped would be a “miracle cure”.
But, on Monday, doctors at Royal Derby Hospital asked for permission to turn off his life-support machine.
The youngster had a form of Leigh’s disease, a complex, life-limiting condition which affects the brain and nervous system.
There is no known cure and it causes weakness, lack of muscle tone and breathing problems. Leo died of a chest infection caused by the condition.
Kenny, 35 of Derby, said: “They turned his breathing apparatus off. At 12.45am they put him in my arms and at 12.46 he was pronounced dead.
“He was fantastic. He had the smile of an angel. But he was fighting a battle he couldn’t win and now he’s in Heaven.”
Leo hadn’t been able to eat solid food since 2007 and had been fed through a tube in his stomach.
He had to wear nappies, was regularly sick and often cried because he was in pain.
From February onwards, Leo had been bed-bound.
Kenny said he would have “cracked up” had it not been for the Swiss drug giving him and his family hope.
He said: “We worked out on the internet that he had six months to live after he was diagnosed on June 6 last year. I just remember welling up and thinking I couldn’t let that happen.
“I made calls all over America until I found the only man in the world who had been working on the condition Leo had, which was a rare form of Leigh’s disease.
“Medical scientist Guy Miller, based in Atlanta, asked if we wanted to trial a drug made in Switzerland and we jumped at the chance.
“It could have extended his life by a few months but I think the main affects were psychological; it gave me hope as they thought it could be a miracle cure that prolonged his life for a few years.”
Kenny and his wife, Kelly, 21, had tried to make their son’s cruelly-shortened life as happy as possible. They raised £2,500 to make a “sensory room” with bubble walls, a projector screen showing underwater scenes of fish and a plasma screen TV.
The cash came from fund-raisers at local pubs and a 100km sponsored bike ride from Derby to Skegness which Kenny and his family completed.
The year 2009 was a busy year for Leo, with visits to Gullivers Kingdom in Matlock; Nottingham Road fire station in Derby; Butlins at Skegness; and Twycross Zoo.
Last year, Kenny quit his £20,000-a-year job as an industrial engineer to spend more time with his family.
He said the move had been well worth it. “It was the best thing I ever did. It was phenomenal to be with Leo for his last eight months.”
Leo’s funeral will be at Nottingham Road Cemetery Chapel on Friday, June 4.