Verification Required

My 13 year old son is on family sharing with me. We just upgraded his phone to an iPhone 7. He has been unable to verify our family’s payment method when trying to do in app purchases on Fortnite or Clash Royale. His phone keeps sending mine the verification code which he has entered correctly dozens of times but it still won’t verify the payment method. Surprisingly, he is able to purchase songs and other apps. Just the in app purchases are a problem. My other two sons have no problem with this. Please help.