How do I prevent my son from making purchases?

They are doing this by signing in to you sons account on their console. I would suggest changing the password immediately and setting up two step verification to prevent this from happening. Please be aware that if there are no barriers on making purchases
on your sons account, the person responsible for signing your son out will also be able to make purchases on the Microsoft store using any payment details linked to your sons account.

Hope this helps

I can try asking but I doubt there is anything they can do.

How do I prevent this from happening in the future? Is there a way to close my son's old account?

After entering in the passcode 3 times it will then ask for the password for the account to reset the passcode the best thing to do is make a password that your son will not know. Or even better yet make your son a child account that has restrictions on
making purchases on the account and this would prevent it in the future.

Under most circumstances there are no refunds for digital content or in game points/funds even when it is purchased by your son. Put a passcode on your account and stop him from making purchases without your approval to prevent it from occurring again but unfortunately I think this is gonna have to be a learning experience.

Thank you for the suggestions.

In more detail to what I am wanting to do:

I have a passcode set up for the family-timer so that my son can play on the device for a period of time. He does not have this passcode.

I would like to prevent him from making purchases (i.e. skin packs for Minecraft or other games) as this has happened in the past.

I would still like for him to have access to live account as they chat and such.

I would like to prevent purchases.

Will try the examples above.

Super thank you

Anthony