He's a rapper from London:
According to The Times of London and other sources, "John" is believed to be Abdel-Majed Abdel Bary, a 23-year-old man from London, who went to Syria to join Islamist forces last year. His father, Adel Abdel Bary, is an Egyptian-born man who is accused of taking part in the 1998 U.S. embassy bombings in Tanzania and Kenya. He was extradited from the U.K. to the U.S. in 2012 and is currently awaiting trial. Bary was an aspiring hip-hop artist who performed under the name L Jinny when he still lived in London. His music has even been played on BBC Radio and there are several YouTube videos of his performances online. It's believed those videos could have been used to match Bary's voice to the voice of the man in the video of Foley's beheading.
Foley's heartbreaking final letter home to his family—which was smuggled out of prison by a fellow hostage who committed it to memory—is here.