His father is thought to have travelled from Australia to Barcelona to join the search for his son, while his aunt and uncle are also thought to be Sydney residents.
Update 3.30pm: Spanish police say reports that seven-year-old Julian Alessandro Cadman, who has been missing since Thursday's terrorist attack in Barcelona, had been found alive in hospital are not true.
The family of a British-born boy who went missing during the terror attack is asking anyone in Barcelona to look for him.
Meanwhile, relatives of Julian Cadman, aged seven, have launched an online appeal to find him after he became separated from his mum Jom during the chaos. Please like and share.
"Julian is 7 years old and was out with Jom when they were separated, due to the recent terrorist activity".
Foreign Minister Julie Bishop said four Australians were injured in the attack including two Victorian men who have been released from hospital.
"This morning I spoke with Mariano Rajoy, the Spanish prime minister, to offer my deepest condolences after the bad attack that took place in Barcelona and the United Kingdom stands shoulder to shoulder with Spain in confronting and dealing with the evil of terrorism and I have offered any assistance that we can provide".
At least 13 people have been confirmed dead and hundreds of others are injured after terror attacks in Barcelona.
'Small number' of Britons injured in Barcelona terror attack
Australia said one of its citizens is unaccounted for, and Spanish newspaper El Pais said three Germans and a Belgian are among the dead.
A woman was also killed in Cambrils after a similar vehicle attack.
Separately, six civilians and one police officer were hurt when a auto drove into people in the town of Cambrils in a second attack.
Police were pursuing a suspicious van that then drove into a pedestrian area on an esplanade, running down six people before the vehicle overturned.
Firearms were removed from the scene and belts they were carrying are being treated as potential explosive devices.
Britain's Foreign Office said that a "small number" of Britons were hurt during the attacks.
Fifteen of them are in a critical condition.
At least 100 people were injured in the attack claimed by Islamic State, while Ms Bishop said Catalonian authorities put the death toll at 16.