Ahmed Muthana, 57, had told the Daily Mail newspaper that a man in the video announcing the death of US aid worker Peter Kassig "looks like my son", 20-year-old Nasser.
But he later told the BBC, after looking at the pictures again: "It doesn't look like him, much difference. This one's got a big nose, my one has a flat nose."
The video showed Kassig's severed head as well as the beheadings of at least 18 men described as Syrian soldiers.
Nasser Muthana, from Cardiff in Wales, appeared in an IS recruitment video in June, and has also been joined in Syria by his 17-year-old brother Aseel, British media reports said.