Paul Bellany, speaking after the death of his father, who is recognised as one of Scotland's greatest post-war painters, said the moment seemed to come quickly.
A paintbrush was slipped into the 71-year-old artist's hand just before he died, Mr Bellany said, and he gripped it "as if to steady himself for where he was going next".
The artist, who was born and raised in the fishing community of Port Seton in East Lothian, a background that was the key influence on his work, died after a long period of illness.
He had been taken to hospital at Christmas with pneumonia and his family believe a heart attack led to his death.
Paul Bellany, a film-maker, said his father's hospital bed had been moved to the middle of his paintings studio at his home in England, and until earlier this week he had been painting watercolours.
Bellany, known for his often turbulent images that frequently featured boats, fishermen and fish, trained at the Edinburgh College of Art and then in London.
His works were exhibited and collected around the world, and were celebrated in a lauded retrospective at the National Galleries of Scotland (NGS) last year.
Paul Bellany said: "The family are all devastated. Fortunately, we were all there for his last moments and when the end came it seemed to come quick. His hospital bed was situated in the middle of his studio and up until a couple of days ago he'd managed a few watercolours.
"It's been a month or so since he painted his last oil painting but as he neared his last breath a paintbrush was slipped into the palm of his hand and he gripped it as if to steady himself for where he was going next."
The painter was planning to do more work, he said, and had been particularly inspired to paint after witnessing Scotland's recent football match with England. He was a keen follower of the game.
In his final days, the painter spoke often about his show at the NGS, especially its opening night.
Of his mother and Bellany's wife Helen, Paul said: "She, like the rest of us, is devastated. She absolutely adored him. Three or four times my father said to her 'I can't bear it that I won't be able to see your face again.'"
The Bellany family believe his funeral will be in one of the cathedrals or churches in Edinburgh early next month.
Bellany had suffered from health problems for years, including a liver transplant 25 years ago, three heart attacks, pneumonia and a long period of depression.
Tributes to Bellany have been led by First Minister Alex Salmond, who said he was one of Scotland's finest artists and an outstanding human being.
Jilly Dobson, the managing director of Bellany's gallery in Edinburgh, Open Eye, said: "John was a generous, kindhearted and passionate man who will be sorely missed in the gallery and by all in the art world."