The official, who has close ties to intelligence services but was not authorized to speak publicly about the inquiry, also said another French fighter whose death in a suicide bombing was announced this week by Islamic State is a young teenager.
The man in the video, Muhammad Musallam, released late Tuesday, speaks with a southern French accent and looks like the step-brother of Mohammed Merah, who killed seven people in attacks on a Jewish school and paratroopers in the south of France beginning on March 11, 2012 – exactly three years ago. In the video, Musallam praises attacks on Jews "in your own stronghold in France" as he and the boy stand behind the man about to be killed.
In addition to the 2012 killings in Toulouse, Jews in a kosher supermarket were among the targets of three days of terror in the Paris area this year that left 20 people dead, including the three gunmen.
"Here are the young lions of the caliphate," the man says in the video. Soon afterward, the child is shown shooting 19-year-old Muhammad Said Ismail Musallam, from East Jerusalem, in the head.
Said Musallam said ISIS lured his son with promises of women, money and cars. "They promised a lot of things and then I came here and there is nothing," Musallam recalled his son saying in a phone call home from Syria.
Ahmad Musallam, brother of Muhammad, told Ynet he had not seen the clip. "May God take care of them," said the tearful brother.
"My brother was a little boy who did not care for religion or politics. Daesh used him and tricked him," he said, referring to the Islamic terror group by its Arabic name.
French government spokesman Stephane Le Foll described the video as a "dreadful" killing, but refused to comment Wednesday on the nationalities or identities of the man and boy.
The French official with intelligence ties also confirmed the nationality of a French fighter whose death was announced this week by the Islamic State group. The group said Abu Ali al-Firansi died in a suicide bombing in Tikrit. The French official described him as a young teen.
The Interior Ministry declined to comment on the video.
Interior Minister Bernard Cazeneuve said in May Merah's sister Souad was likely to have travelled to Syria with her children.
About 1,400 people, including entire families, have left France to join extremists in Syria and Iraq, and many have returned. Security officials fear some will arrive with honed skills as fighters, and with passports that allow free travel.
"Nearly 90 French citizens have died there, weapons in hand to fight against our values," French Prime Minister Manuel Valls told France's iTele on Sunday.
About 3,000 Europeans are part of the fight, Valls said, adding: "There could be 5,000 before the summer and without a doubt 10,000 by the end of the year."
Roi Kais, Elior Levy, and the Associated Press contributed to this report.