Jon Philo, of Hermitage, Berks, was at North Yorkshire’s Lightwater Valley theme park with his son when he saw the “scary sight”.
He vaulted over the barrier to help the boy along with an off-duty police officer to make sure the little lad was alright.
Jon said the boy’s mum screamed and helplessly looked on as the ride continued to move.
When the dad arrived the lad was lying motionless on the ground and waited by the child’s side until paramedics arrived.
The youngster suffered facial injuries in the horror plunge and was air-lifted to Leeds General Infirmary.
Later, Jon and the boy’s family received the good news he didn’t suffer life-threatening injuries and had recently left the hospital.
Jon later said: “My son and I tried out Lightwater Valley theme park queuing for the Twister rollercoaster ride when we both saw a young boy fall about 30ft from the ride.
“Shocking scenes, but I had no other thought than to jump over the fences and climb over the rollercoaster tracks with the coaster still in motion to be the first on the scene.
“My first thought after seeing him was not good, as he was motionless. Very scary moment”
“I was followed by an off-duty police officer and we were shocked by what we saw. The boy, who was severely autistic and non-verbal, had facial injuries which I won’t forget in a hurry.
“My first thought after seeing him was not good, as he was motionless. Very scary moment. I have to say the officer and his wife were amazing. I was very proud of my boy for helping the paramedic guide the air ambulance where to land and his patience while I was with the boy for over an hour.”
The lad’s horror fall comes 18 years after student Gemma Savage, 20, died when two cars on the same Twister ride – then called Treetop Twister – collided.
Light water Valley describes the ride on its website, saying: “The track is full of seriously tight turns, giving riders the impression that they might not make it around the next corner, with the threat of plummeting into the treetops being a constant source of tension for parents (and amusements for the kids).”