Minor with negative credit items on credit report

My nephew lives with me and my wife and has lived here for 3+ years now. We are his legal guardians. When he was with his mother for about a month (reunification is the term they used) he had to have a couple x-rays on his foot. His mother didn't pay the bill and the doctor turned the bill over to collections. He was 15 years old at the time and just recently he wanted to get a checking account and was turned down because of negative credit items. We ordered a credit report and see that this all stems back to when he was with his mother. I have tried to talk with the collection agency and they refused to talk with me about his/his mothers case. I don't understand how they can hold him responsible for this debt? can anyone explain this to me or point me in the right direction?

I don't see it being legal to hold him responsible for this since he was a minor. I would call the original creditor and advise them of this. I would then have him write the credit bureaus and dispute this as a fraudulent account. He was not of legal age to enter into any type of legal contract,oral or written.