Abdullah Deghayes, 18, from Brighton, is believed to have died in Kassab, in Latakia province, earlier this month after leaving the UK in January.
Abubaker Deghayes, who learned of his son's death last Monday via Facebook, said his two others sons, Jaffar, 16, and 20-year-old Amer have also travelled to Syria.
He said: "Amer, Jaffar, if you see me or this interview please, please come back home. Enough. This war has taken Abdullah already... I'd like to see you live longer."
Mr Deghayes revealed he travelled to Turkey earlier this year to meet Abdullah and Jaffar in an attempt to stop them entering Syria.
He said Amer had joined the jihadist group Jabhat al-Nusra. "I am scared for my children," he said.
Speaking at his home in Brighton, Mr Deghayes said his son had died a martyr in a battle which saw Amer suffer a bullet wound to his stomach.
"Mr Deghayes insisted his three sons were not "terrorists" but travelled to Syria to defend "those who are weak".
Abdullah, who was due to go to university in Brighton, is the nephew of Omar Deghayes, who was held by the US at Guantanamo Bay from 2002 to 2007 after he was arrested in Pakistan.