What things could a high school graduate do in a gap year between high school and college?
Go to community college, take different classes and see which subjects he likes.
 
 Travel / backpacking in a very different part of the world.
 
 Get a job, earn some money and get work experience.
 
 Take on a passion project and try to make it work.
 
 Volunteer at a non-profit in an area he might want to pursue as a career.