What you want to do is to set short term achievable but challenging goals for him. Never set anything that is out of reach or too difficult. When the goal is reached, a reward is given.
At this point, fun, and a few rewards should be enough to motivate him, which from your point of view should be your number one goal. Be patient with him, you'll need it when teaching youngsters. :p
In order to get him to full draw, forget about the bow. Teach him the basics without it or with a theraband (surgical tubing), and reward him for his accomplishments. At a certain point he will be able to transfer what he has learned to his bow.