Grace Dare, who was brought up in the UK by a Christian family of Nigerian descent before being radicalised online as "Khadija", is the mother of the little black boy who is seen pointing into the distance and calling for the killing of "unbelievers" in the 11-minute film released yesterday.
Dare's London-based father Henry Dare today confirmed to 'The Daily Telegraph' that the boy in the video is his grandson Isa Dare.
"I was surprised when I saw the picture. It's definitely him. Of course I'm worried but there's nothing I can do now. I'm not angry, I would never have expected it. I just hope someone is trying to bring them back," the London taxi driver told the newspaper.
Grace Dare, an Islamic State member who has in the past expressed a wish to become the first woman to behead a hostage, made her way to Syria in 2012 and married a Swede known as Abu Bakr, who is thought have been killed since.
Her son, dubbed "Jihadi Junior" by British media, is dressed in camouflage and a black headscarf with an ISIS logo on it in the new video.
Last July, 22-year-old Grace "Khadija" Dare had posted a picture on social media of Isa aiming an AK-47 automatic rifle that he seemed only just big enough to lift, the newspaper claims alongside the photograph.
Asked about the police response to his daughter's radicalisation, Henry Dare said: "I said they should watch her. She was a Christian, she went to the local Roman Catholic school. She was Christian until she changed. She fell under the influence of some people when she was going to the mosque. She was very observant."
His wife, Victoria, has earlier spoken about how her daughter changed her first name to Khadija after she began attending the mosque.
She said last year: "I still call her our Grace. I want her back in my life. She is the only child that I have and the devil took her away."
Khadija has since appeared on television documentaries about jihadi brides, saying she missed junk food and Chinese takeaways, but said she would never return home.
She used social media to gloat about the beheading of the American journalist James Foley and said she wanted to be the first British woman to kill an ISIS hostage.