Ibrahim Iqbal posted pictures of himself in full combat gear and holding an AK-47 on the social media site.
Pictures triggered fears that ISIS may be using him to lure other western recruits.
"We are aware of posts made on social media. These are currently being reviewed to establish if any offences have been committed," the UK's North East Counter Terrorism Unit said in a statement.
Ibrahim was reported missing by his father from their home in Bradford in June last year when he disappeared with his four siblings, aged between 3 and 15.
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His mother, 35-year-old Sugra Dawood, is believed to have left to join her two sisters, Zohra Dawood, 33, and Khadija Dawood, 31, who took their own children with them.
Iqbal's uncle, 23-year-old Ahmed, had reportedly already been fighting for ISIS for about a year.
Iqbal's recent post made references to his brother Junaid, 16, and uncle Ahmed Dawood, 23, with the words: "May Allah accept them," a common Islamic prayer to the dead indicating that they might have been killed.
Ishtiaq Ahmed, a spokesperson for Bradford Council for Mosques, told 'Daily Mirror': "We are very concerned. If it is confirmed that these pictures are real then it will raise additional alarm bells that young people are being used by the so-called Islamic State to propagate their violent game and entice other young people from Britain to join."
It is believed that 10 members of Iqbal's family travelled to Istanbul on a flight on June 9 last year and the group split into two groups to cross into Syria from Turkey, an ISIS smuggler had later revealed.