IRS is auditing my 2017 1040A and is asking for supporting documents to prove my 15 year old Step-grandson live with me. Been living with me since he was 5. What do I do?

Why is the IRS auditing me to show proof my step-grandson live with me after all these years. He's 15 now. He started staying with me when he was 5. They said in letter I need to include a copy of his birth certificate and a copy of his parent birth certificate to prove the relationship