Our teachers use a technique for studying that may work for your son. They put the questions, math problems or vocabulary on index cards (question on one side, answer on other). They have the student go through them as quickly as possible. When the student answers correctly, the card is taken from the stack so that they are only practicing things they cannot answer correctly. They time how long it takes each time to get through the stack with correct answers. It becomes a game to see if they can do it faster. Perhaps you could do this with your son with a goal of three times through, faster each time? This way, you make up the game instead of him! He should be allowed to do something else if he can answer them correctly. No need to drill things he already knows.