I have an Autistic grandson and I bought him an iPhone so I can always locate him. I had to put a credit card on the phone to be able to load the apps we wanted on the phone even though they were free. What can I do about his charges?

Since I did not know that he could then buy music and so forth on his phone, he has charged thousands of dollars to my credit card. He can’t even read or write, so how do they set this up to charge for things automatically? I thought I could stop this by changing the credit card number, but this did not change anything. Now I owe on two cards. I have now bought a refillable credit card for him and put this on his iPhone. But this automatically charges extra for everything he does, so it eats up the money quickly. What can I do about this to help my family?