My question involves paternity law for the State of: Montana

I am 38 and my father has never been on my birth certificate. He raised me and never crossed anyone's mind that he needed to be on their. He was the one who enrolled me in school, insurance, everything he needed to do he has done, all without his name on the certificate, and it has never been a problem. My mother has rarely been in my life, how do I go about adding his name now?