He cannot move out without his parents permission unless he is emancipated (the courts say he is no longer a minor). He would need to prove he can support himself on his own, food, board, rent, everything... and it costs money, so it is probably easier just to wait until he hits 18.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship. This attorney is licensed in Texas.
He can move only with the permission of his parents. His parents have the control and say-so about where he lives until he is 18 years old. If he leaves home without their permission before age 18, he can be reported as a runaway and picked up and returned home. If your family provides him with housing knowing he is a runaway, they can be charged with harboring a runaway.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.