The boy originally appeared in a video posted to sites such as YouTube in 2012:
Here is the clip from the Islamic State video in which the little boy, now older, is said to appear:
Of course, it is difficult to know with absolute certainty if it is or is not the same boy. Twitter users, both affiliated or supportive of the Islamic State and others who are not connected but who are close watchers of the conflicts in Iraq and Syria at all have said it is, while others have said it is not. Regardless, the fact is that the environments and situations in Syria and Iraq have led many youth and others who would normally not join groups such as the Islamic State or Jabhat al-Nusra (or other armed groups) to become gradually socialized to accept higher levels of violence and radicalism. This social effect has been seen in many other cases, such as the conflict in Northern Ireland. Many "militants" are not born "militant" and extreme. They are socialized and shaped by the environments and situations in which they (are forced to) exist.
There are a couple other similar cases of young men who started out either not supporting the uprising or who went from supporting one of the Free Syrian Army(FSA) militias to more radical groups like Jabhat al-Nusra: https://youtu.be/AFZhQ4u2Cbg?t=4m13s (this young man later moved over to Jabhat al-Nusra)
The young man followed in this PBS Frontline documentary also goes from supporting one of the FSA militias to Jabhat al-Nusra over time: http://www.pbs.org/wgbh/pages/frontline/syria-behind-the-lines/