Jack Letts travelled to Syria when he was 18 after converting to Islam and telling his parents he was going to study Arabic in Kuwait.
A keen footballer and good student, admitted to his parents that he was with ISIS in Syria in September 2014.
While Letts's parents, who live in the city of Oxford, declined to comment, a source close to the family told 'The Sunday Times': "His mother and father were extremely worried for his safety after he told them that he was in Syria. The past two years have been a real nightmare for them. They just wish he can be back home with them."
His father, John, is an organic farmer and archaeobotanist who specialises in growing and milling heritage grains. His mother, Sally, is a former books editor.
It is understood that Letts married a woman from the Iraqi city of Fallujah shortly after arriving in Syria. His decision to join ISIS has surprised former friends at Cherwell School, Oxford.
"He was very funny and fun to be around," recalled one former school friend, who said Letts had now been nicknamed "Jihadi Jack" by some of his peers.
"At school he was very much the classroom clown and was liked by a lot of the students. That's why this whole thing of him going to live in Syria and join ISIS doesn't make any sense," the friend told the newspaper.
Letts is among more than 750 British men and women who have joined ISIS in Syria and Iraq. 'The Sunday Times' said last week that up to 100 have been killed but there is no evidence that Letts has been involved in any fighting.
Letts, who now reportedly uses the name Ibrahim, is believed to be living in Raqqa, the ISIS de facto capital in Syria and a target of US-led airstrikes.