Typically in high school social studies, students cover:
US History
World History (Ancient to Modern)
Civics (Government)
Economics
But you could also branch out to things like:
Current events
Human Rights
Cultural studies
Philosophy
Sociology
Depending on what your teen has already studied, pick a "spine" book in one of these topics and then choose a chapter or two to get started. Read the chapter, but then move out of the book and into YouTube videos, documentaries and biographies. No one can cover everything, so get comfortable now with excluding things - especially ones your teen isn't interested in! Crash Course has many free videos in US and World History, Economics, and Sociology to supplement your learning. Go through some of the lessons on iCivics (free) - such as the Foundations of Government series
Or sign up for some more complete online studies, such as CK-12 History, Global Academic Productions (self paced or live classes), or Time4Learning High School History. Outschool has many short or full semester live history, economics and government classes as well.
For more options, see the Curricululm Pages.