The video identifies the man as 19-year-old Mohamed Said Ismail Musallam, an Israeli citizen of Palestinian descent.
In the video, ISIS shows Musallam's Israeli passport and claims he's an agent sent to infiltrate the group. The 19-year-old's family told CNN Tuesday that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.
"Mohamed told me and his brother that ISIS took him," according to Said Musallam, his father. "They sent him money through the Western Union. They said you will have girls, money, cars, villas, paradise, but afterwords he discovered that there is nothing."
It wasn't long before Musallam's family members didn't recognize him when they talked to him on Skype. The man they knew as a kind and funny brother and son who was once a volunteer firefighter had grown a long beard and was carrying a rifle.
His father tried to help him get home, sending him money and even enlisting the Red Cross. But his son never made it back to Israel. About a month ago, Said Musallam said, he was told his son was taken by ISIS when he was on his way back and trying to cross the border.
A video posted Tuesday on ISIS-affiliated social media accounts shows a man who appears to be Musallam on his knees, wearing an orange jumpsuit.
An adult ISIS fighter and a child -- both in fatigues -- stand behind him.
The adult, speaking French, gives a command to the child to go forward with the killing. The child steps in front of the man and raises what appears to be a 9 mm semiautomatic handgun and shoots Musallam in the forehead. The man immediately falls forward to the ground. The child appears to then fire at least two more shots into the body.
'I used to take care of him'
Ahmad Musallam said he'd seen the video of his brother and was devastated by his death.
"I used to take care of him and babysit him when Mom went to work," he said, crying. "Mohamed is not a brother. He is a son."
But he refuses to show the video to his parents, who still can't believe their son is dead.
An issue last month of ISIS' English-language propaganda magazine, Dabiq, included a purported interview with Musallam and described his alleged work for the Israeli spy agency.
In the ISIS video, Musallam seems to be reading what appears to be a prepared confession, saying he is an Israeli intelligence agent working for Mossad, sent to infiltrate ISIS.
Musallam's family members said they believe he was coerced in the video, forced to lie about ties to Israeli intelligence.
"Mohamed is not an agent. Mohamed doesn't have a shekel. If he was an agent he would have lived a beautiful life," his mother, Hind Musallam, said. "We could have been living a different life and I would not be working cleaning houses so we can live."
ISIS propaganda features children
An ISIS propaganda video released in January -- one that CNN could not independently verify -- shows a boy with a pistol apparently shooting two men
in the back of the head. The boy then stands over one of the bodies, fires two more times, and later raises his pistol high. Last August, a photo posted to Twitter from an ISIS stronghold showed a 7-year-old boy holding a man's severed head and his father's words, "That's my boy."
ISIS has featured children as fighters before, calling them the "cubs of the caliphate" (the adult jihadis call each other "lions") and has encouraged foreign fighters to bring their families.
It has taken over schools to indoctrinate children. Human Rights Watch says ISIS and other extremist groups "have specifically recruited children through free schooling campaigns that include weapons training and have given them dangerous tasks, including suicide bombing missions."