The sentence against Ali al-Nimr, a Shiite Muslim who was only 17 when he was arrested in February 2012, has drawn international condemnation over his young age and allegations that he was tortured into making a confession.
"When I visited my son for the first time I didn't recognise him," Nusra al-Ahmed told the newspaper.
"I could clearly see a wound on his forehead. Another wound on his nose. They disfigured it. Even his body, he was too thin.
"For a month he was peeing blood," she added. "He said he felt like a mass of pain, his body was no more."
In an interview with AFP last month, his father Mohammed al-Nimr said he hoped the king would save his son and warned that if his son is put to death the minority Shiite community could react violently.
Mother Nusra al-Ahmed called the sentence -- which she said would involve her son being crucified after he is decapitated -- "backwards in the extreme".
"No sane and normal human being would rule against a child of 17 years old using such a sentence. And why? He didn't shed any blood, he didn't steal any property."
She called on Obama to exert his influence on the Saudi authorities.
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"He is the head of this world and he can interfere and rescue my son," she said.
"If he carried out this act, I feel it would raise his esteem in the eyes of the world. He would be rescuing us from a great tragedy."
The youth is a nephew of Nimr al-Nimr, a Shiite religious leader who is also on death row having been identified by Saudi authorities as a driving force behind demonstrations that began four years ago in Eastern Province.
Most of Sunni-dominated Saudi Arabia's Shiites live in the east, and have complained of marginalisation.
Ali al-Nimr's father admitted that his son, then a high school student, had joined thousands of other people in the protests.
But he insisted that Ali was innocent on numerous other charges including burglary, attacking police and using a Molotov cocktail.
The court sentenced Ali al-Nimr to death but gave no further details.
Execution in the kingdom is usually carried out by the sword, sometimes in public.