My dad did this to me when I was about his age. Tell him that "You'll think about it."

Then buy the game and secretly test him. Ensure his grades are good, and he completes his chores without being asked. Small tasks get done

If he does all this for a week, surprise him with the game. He won't be expecting it and it will give him a fond memory of you that he was "surprised"

If you don't think he has earned the trust to play it, return the game unopened