I want custody of my nephew in another state. He is in Florida and my family is in Maryland. What should I do?

His mother will not give up full custody. I had him all last year on a power of attorney but I cannot put him on my insurance with that. I have read about temporary custody but where would I file? She has another child so I don't want to call child services on her. My nephew does better in school here and is more disciplined. Guidance please.