From WND:

The preacher, apparently about 9 or 10 years old, is identified as Abu Jafar.

His message: “Make haste and purchase your place in paradise, my dear brother. Go to the battlefields of jihad and kill the enemies. Kill [Syrian President] Bashar al-Assad and the enemies of Allah and of our religion. Destroy them, and if you die, you will become a martyr.”