It's interesting you mention this, because there is actually a distinct international-law-based crime recognized under the Rome Statute for persons who employ those under the age of 15 as child soldiers. With child soldiers between the ages of 15 and 18--even though under the Optional Protocol they're deemed to be essentially involuntary participants--it does not appear to be a separate crime under international law to employ a child soldier.
Now, the protocol does call upon state parties, such as Canada and the United States, to have national legislation that would make it a crime to employ persons under the age of 18 as soldiers. I don't know whether Canada has such a statute or such a provision, but with respect to the question of prosecution, clearly his family, and ultimately his father, should be deemed responsible. Unfortunately, I should say, his father is dead.