My son paid for an app can I cancel it and get a refund.

In some countries, App sales are final.

After making an App purchase, you should receive an email invoice within a few days - and this will, in many circumstances, include a link to “report a problem”. If a refund can be requested, you’ll be able to use this link to apply for a refund.

You should note that unless you have statutory rights to a refund, your application to reverse the transaction will be at the sole discretion of Apple.

On the receipt for the app there should be an option for refund. If not then you may have to check under purchases for the account to see if that option is available.