Start at your son's school. Talk with his teachers and his school counselor to find out the areas of study where he needs help. Often teachers and counselors know of retired teachers who enjoy tutoring, or perhaps there are teachers in your son's school who tutor students after school hours. The school staff can also help you know how much tutors in your community are paid per hour.
If there is a college or university near you, call the appropriate department and ask if they have a list of students willing to tutor high-school students. If they don't, ask if they would be willing to let you put a notice requesting tutoring help on their bulletin board.
Also check you local yellow pages for private tutors or tutoring agencies in your community. Most are very happy to provide references and contact information for former clients with whom you can check on their services.