George Tyson, 61, was killed almost instantly after he pushed Garry out of the path of the oncoming car but took the full impact himself.
His 32-year-old son was airlifted to hospital and later discharged after being treated for minor injuries and shock.
The pair were often together and regularly went on long walks. It was as they took a bracing stroll along a coastal road near the village of Bardsea in Cumbria on Sunday afternoon that the tragedy happened.
At about 4pm they were at a road junction when at the last second Mr Tyson spotted a Vauxhall Corsa hurtling towards them and managed to push his son out of the way.
Last night Mr Tyson's distraught family praised him for making the ultimate "heroic" sacrifice which saved the life of his son.
Melanie Tyson, his daughter, said: "My dad's life was cruelly taken away from him, although through various witness statements, the story is blessed with his selfless act of saving the life of my brother.
"We, as a family, are still in shock but realise that my dad's quick action saved it from being an even bigger tragedy."
Mr Tyson, a carpenter from Ulverston, Cumbria, was devoted to his son, who was left with learning difficulties after suffering brain damage in a fall when he was 18 months.
His daughter said: "Most people of Ulverston will know him through the care and help he has shown my brother, and most people with have at some point seen them pacing through town on their many missions.
"Garry was inseparable from his dad and the huge amount of time, care and patience my dad showed will be hugely missed.
"My dad died doing the thing he loved the most – he was walking with my brother Garry."
The road was closed for several hours while road traffic police officers carried out their investigation. Cumbria Police are appealing for witnesses.
A 17-year-old from Barrow-in-Furness was arrested on suspicion of causing death by dangerous driving and bailed until April 4 pending further enquiries.