my credit card keeps getting declined on my sons ipad

It's a 'proper' credit card, not a prepaid card ? And its details match the details on the iTunes account ( If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - Apple Support ), and was issued by a bank in the country where you are (and therefore the country which is on the iTunes account) ?

Are you entering your credit card for an Apple ID under his name?

If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - https://support.apple.com/HT203005

Check that the billing information was entered correctly. If so, contact your bank or the iTunes Store staff.

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