Ryan Lock from Chichester in West Sussex had travelled to join the People's Defence Units (YPG), the Kurdish military force fighting in northern Syria in August last year. The chef had no military experience.
In a letter seen by the BBC, the YPG said he was killed on December 21 2016 in Jaeber village in the battle for the Syrian city of Raqqa.
The letter offered condolences to the family of Ryan, given the Kurdish name Berxwedan Givara.
It added that while Ryan's family had lost "their beloved son" and "we the YPG lost our daring and courageous companion".
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"Ryan was a very caring and loving boy who would do anything to help anyone. He had a heart of gold. We ask for privacy to allow our family to grieve," his father John Lock said.
Lock is believed to have joined Kurdish militia after telling his family he was going on holiday to Turkey.