Find a charity he likes,

 

e.g., if he likes animals, he can collect money for a local animal shelter.  You donate money to his collection each time he solvles more math problems.

When the money reach to certain level, say $200, you can take him to that shelter, ask the clerk there how they take donations.  If they take pet food as donations, you can take him to buy the food.

Then he can have a connection between his effort and a reward that does not corrupt.

You can tell the clerk that you are considering adoption and take him to visit the animals.  That can be a good experience for him even if he is not interested in animals.