Start with not telling him to do it. Bring him into the process, negotiate general schedules with the presumption that he wants to work on the material.
Like: "I'm planning on shopping this afternoon, when did you want to work on your fun school challenges so I can fit that into our schedule."
Or have him decide a general schedule: "How many days this week do you want to work on your learning?" and "OK, how long do you want to set aside for it?" and "Did you want me to remind you when it is the time you chose to work on it or manage your own schedule?"
Be the partner and guide rather than the boss; so, that he owns it along with your confidence and trust.
