"This morning I spoke with Mariano Rajoy, the Spanish prime minister, to offer my deepest condolences after the awful 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".
His mother Jom was located in hospital in a serious but stable condition.
Relatives of a seven-year-old boy missing after the Barcelona terror attack have flown to Spain in their desperate search for him.
Mr Cadman is understood to be British, and Julian - a student at St Bernadette's Primary in Lalor Park - may have dual citizenship by descent, according to media reports.
Julian Cadman, 7, became separated from his mother Jomarie "Jom" when the vehicle rammed pedestrians.
The appeal concluded: "Please share, especially if you have family or friends in Barcelona".
The youngest victim is believed to be a three-year-old toddler from Spain, who died alongside his 60-year-old great-uncle. Please like and share.
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In the photograph, Julian is wearing a jumper which has a crest that says "Chiddingstone Nursery".
After Mr Cadman made a plea for information, Prime Minister Theresa May said United Kingdom authorities were "urgently looking into reports" that a child, who has dual British nationality, is missing.
There are now calls for barriers to be introduced in the Las Ramblas area, where it happened.
Eight hours later terrorists strike again, this time in Cambrils, a coastal town and resort 68 miles down the coast from Barcelona.
The Foreign Office said a "small number" of Britons were among those hurt and extra staff were being sent to Barcelona to help.
Police shot dead five suspected terrorists at the scene.