Is there a way to set up a password for purchases?

Hello!

You should be able to go on the Xbox, Go to settings and account. There will be an option for security & passkeys, then create a passkey on the account. Then
in the same settings area change the preferences to require the passkey any time a purchase is made.

Here's the link to the page explaining how you go about creating a passkey that will be needed to make purchases on your account in the future.

https://support.xbox.com/my-account/security/passkey-purchases

You should have that set up on your account and your son playing on his account without any payment information on his account.

You can create a family account and set time limits, purchase restrictions and all sorts of stuff that will suit your needs best. The instructions to do that are here

If they don't know the password and you set up your account so that the password needs to be entered for all purchases, then you should be fine. I have my account set up that way and, every time I want to make purchases, I have to enter my password