In this situation, there are 2 possible avenues to pursue here. One is that your could petition the court to become his legal guardian. A legal guardianship is a formal decision by a judge that suspends parents' custody of their child and gives custody to a non-parent. Typically, any adult 21 or older, can serve as a legal guardian for a child under 18.
A legal guardianship gives an adult who is taking care of a child the formal authority to provide for the child's needs. They need not worry about being charged with "kidnapping" or "harboring a runaway." A legal guardianship can be helpful in establishing stability for a child who has come from a troubled family setting. In addition, a legal guardianship may be necessary because of health insurance considerations, or for the minor to be eligible to receive government benefits. A legal guardian has the same responsibilities for support, care, health, education and welfare of a child as a parent does. A legal guardian must feed and clothe the child, provide for the child's education and general welfare. Once the court has granted the legal guardianship, the legal guardian acts as the child's parent. The legal guardian then has the right to raise the child without interference from anyone, including the child's parents.
Another way to legally take this boy in is if he his "emancipated". In most states, someone is emancipated at age 18; this is the age where the law considers them to be an adult. Prior to that a minor is subject to the legal control of their parents. A minor can become emancipated before their eighteenth birthday. One way is obtain a court order granting emancipation. In order to successfully petition the court a minor would have to show, among other things that, they have a safe place to stay, they can financially provide for themselves, and they are stable and mature enough to handle the responsibilities of adulthood. However, courts do not routinely grant emancipation.
At this point, you really need to consult with an attorney in your as to all of this; they can best advise as to specific state law. You have a good heart. Best of luck.