Dean Carl Evans, 22, from Reading, died in July in the city of Manbij after joining a Kurdish military group.
His father revealed his will states he wished to be buried with his "brothers and sisters" in the war-torn country.
The Foreign Office has told him his body may still be in Syria despite reports it was moved to Iraq.
John Carl Evans, from Wallingford, Oxfordshire, said his son was shot and then killed in a rocket attack.
He added that the will sounded as if "Dean knew he would pass away there".
"He was shot, and a Kurdish woman tried to help him. But they were fired on by a rocket-propelled grenade round and both lost their lives," he said.
"I was told his body was moved to the Iraqi border and then to Erbil, but now I am told it is still in Syria," he said.
The former soldier said he had no idea his son was in Syria, only learning of his whereabouts when the Foreign Office contacted him to tell him about Dean's death.
"The last time I spoke to him was just before his 21st birthday, but it was hard to get hold of him after that," he said.
"I thought he was a farmer in Wiltshire, living with his stepfather.
"The news still hasn't sunk in, I haven't had any time for my own grieving. But I am proud of him and his family is proud of him."
Dean travelled to Syria to join the People's Defence Units (YPG) - a Kurdish military force fighting in northern Syria.
The YPG described Mr Evans as a "man with the noblest of intentions who planted a seed of love into the hearts of [his] friends and all the peoples of Rojava [Western Kurdistan]".
The Foreign Office said it had advised for some time against all travel to Syria.
"Anyone who does travel to these areas, for whatever reason, is putting themselves in considerable danger," it said.
The father of a man who died fighting so-called Islamic State in Syria says he doesn't know where his son's body is or if it will ever return to the UK.