Julian and his mother Jom had been in Barcelona for a family wedding, and became separated when a van ploughed into crowds in the city's busy Las Ramblas district on Thursday evening.
Julian's mother, who is originally from the Philippines, remains in hospital, one of more than 100 people hurt in the attack that has been claimed by Islamic State.
Julian's father, Andrew, arrived in Barcelona from Australia on Saturday evening (local time) to try and find his son, who is a British-Australian dual national.
Early on Monday morning Australian time, the Department of Foreign Affairs and Trade released a statement on behalf of the Cadman family.
Police have put up scores of roadblocks across northeast Spain in the hopes of capturing a fugitive suspect from the 12-member Islamic extremist cell that staged two vehicle attacks.
Complicating the manhunt, though, was the fact that police have so far been unable to officially identify who exactly is at large.
Police have identified the 12 members of the cell, but three people remain unaccounted for: two believed killed when the house where the plot was being hatched exploded on Wednesday and a suspected fugitive, Catalan police official Josep Lluis Trapero told reporters on Sunday.