Buy a card at the store, set his permissions to download content. Otherwise the points go to you, the account holder. You can then give your son an allowance. He still can't use the points without your permission though. This involves verification. Just
get a card.
Or if on the same Xbox, download for you, he can still play it. If a different xbox, log into your account on his xbox and download it. Just remember you can only transfer content licensing to an xbox once a year. Shouldn't be an issue if you never want
it on your xbox.