Hi Loganrev,
I understand that you're attempting to add to your son's iTunes Store balance, and I'll be happy to assist you.
In a situation like this one, you'll first want to ensure that your voucher or gift card is the right type for the store that you're working with; here's more info: What type of gift card do I have?
After verifying that you have the right type of Gift Card (or if you're using a different form of voucher), you'll want to review this article: Redeem App Store & iTunes Gift Cards and content codes
If neither of these articles have helped, you should reach out to our billing specialists here: Get Support
Cheers.