Contact the state Board of Education. Each of the 50 states has one located in the capital. Ask to be directed to the appropriate person for homeschooling requirements. Ask this person to send you the forms you need to register a homeschool student, and for everything else you need to ensure the student meets the requirements to finish his senior year of high school.
Join a homeschool community. If there are other homeschool families in your area with high school students, spend time with them. Join an online homeschool community for advice, ideas and support. The transition for a high school senior from traditional school to homeschool may prove difficult, and you will need to talk to people with experience.
Consider curricula carefully. Many homeschool curricula offer K-12, but different students and families do well with different curricula. It is helpful to have a homeschool community when choosing curricula, because you can ask other parents for reviews and suggestions.
Develop a routine. A high school senior may lack the self-discipline to establish a routine on his own. Enforce rules such as getting out of bed and starting school at a certain time every day. Have daily goal sheets that the student must complete before school is over. Point out to the student that homeschool is no different than traditional school, where there are established routines.
Go to the library regularly. This gets the student out of the house and into an environment full of resources. Relying upon Internet access at home is isolating, and there are books at the library that are not online.
Stay on top of paperwork. File evidence of advancement and assessments with the Department of Education in a timely fashion so that the student's status of "graduate" will not be delayed.