Apple balance

Are you trying to gift an app? Gifts cannot be purchased with personal balance, only with a payment method such as a credit card.

Send App Store & iTunes Gifts via email - http://support.apple.com/HT201783

"Gifts are charged to the payment method that you have on file for your Apple ID, even if you have a credit balance available. You can't buy a gift with credit."

In addition to Limnos’s question, we wonder what else you have not told us. Examples:

• Is your son a member of Family Sharing? He does have his own Apple ID, right?
• Have you changed countries or regions recently?

What does the message say exactly?