What things could a high school graduate do in a gap year between high school and college?
user
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.
assistant