Shahan Choudhry was also joined by his wife and three young children in the war zone after they left their London flat last year, according to a media report.
Choudhury first came to the attention of British police in 2005 when he was charged with the murder of Karl Hamilton, a 17-year-old hospital worker.
He was accused of being part of a gang of Asian youths who clashed with Hamilton and his friends over an alleged 15-pound drug debt, 'The Sunday Times' reported.
Choudhury, also known as "Spadey", was acquitted at the Old Bailey court in London in 2007. But his family believes he was "brainwashed" by Islamist extremists during the 18 months he spent awaiting trial on remand at Belmarsh maximum security prison in south-east London.
"He was just like any other western kid when he went in but by the time he came out he was going on about religion and how the non-believers would all end up in the hellfire," a relative told the newspaper.
Shahan Choudhury, 30, is believed to have travelled to Syria towards the end of 2015. His 26-year-old wife Mehak is known to have followed him in early 2016 with their two daughters, aged four and two, and their baby son.
The family disappeared overnight, leaving all their possessions at the flat, according to their landlady.
Choudhury is the latest Briton to come to light as a follower of UK-based radical Islamist preacher Anjem Choudary, currently in jail for attracting support for ISIS after avoiding prosecution for almost two decades.