A question regarding adoption and/or legal guardianship...?

My relationship with a single mother ended very civilly after two and a half years. During that time, her son and I became very close and speak at least monthly, two years after the breakup. He is now fourteen and lives with his mom in Tennessee; I live in Denver.

I am in no way trying to take him away from his mother but need to establish some sort of legal ties for several reasons. First (and most important), I am the closest male figure he has had in his life and want him to have a 'father' of sorts (his biological father has been out of the picture since his birth and has a family of his own).

My second major concern is his mother's failing health. She has MS and it is not responding well to treatment. In case of the unthinkable, I need to know that he will be taken care of the way his mother would like him to be. Her parents and the rest of their family have never agreed with some of her choices and have never accepted me as a part of his life. I fear that if her family becomes the primary care giver for him that I will be pushed away and that her wishes for his future will be disregarded.

I know a lawyer will be needed at some point but am gathering as much information as possible. I have not spoken with his mother yet but know from previous conversations that she will be relieved that I want to take this role in his life. At this point, it is almost ceremonial - he will be 18 soon, but a lot can happen in four years.

What is my best option for securing legal rights for the child to ensure that I will be a part of his life and that he will be cared for properly?