Abdullah Deghayes, 18, whose uncle is a former detainee of the Guantanamo Bay camp, died earlier this month after leaving Britain in January.
Abubaker Deghayes, who learned of his son's death via Facebook on Monday, said 20-year-old Amer suffered a bullet wound to his stomach in the battle in which his brother was killed.
He said he had travelled to Turkey earlier this year to meet two of his sons, Abdullah and 16-year-old Jafar, in an apparently unsuccessful attempt to stop them going to Syria to fight.
"He was killed in a battle, as far as I know.
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"His brother, who is also there, is injured. The third brother who is also there is OK. He is fine."
"I never encouraged them, nor anybody, as far as I know, who is around them encouraged them. They went of their own free will.
"Of course I think, as a Muslim, that my son is a martyr. Anyone who dies for a just cause is a martyr."
He said they had been "stubborn" about travelling to Syria after viewing videos of the atrocities online.
The dead man's uncle is Omar Deghayes who was held by the United States as an enemy combatant at Guantanamo between 2002 and 2007 after he was arrested in Pakistan.
A Foreign Office spokeswoman said: "We are aware of the death of a British national and are urgently looking into it."
Abdullah was due to begin studying at the University of Brighton.
He has a twin brother, Abdur-Rahman, who is in Britain. His father said Abdur-Rahman had taken the news of the death hardest. "He has been crying so much... And it's not like him to isolate himself and cry," he said.
It is believed around 250 have now returned to Britain.
British intelligence services have expressed concern about the risk of aspiring jihadis going to Syria to learn how to fire guns and build bombs before using their training to launch attacks on Britain.