I agree with PPs that high schoolers have plenty of time to decide on a major. During my college orientation (back in the dark ages), I remember a speaker telling us that most were undecided, and even those who thought they were certain of their major, would most likely change it along the way.
Let him see how his courses go, and he can explore further along the way. Youtube has college lectures, science podcasts, demonstrations of experiments, footage ranging from deep space to deep sea and everything in-between. Government agencies, museums, publications/media channels, academic organizations, etc. have websites and/or Youtube channels.
Here are some resources that he might find helpful:
Hoagies has links for enrichment for all ages and subjects.
https://www.hoagiesgifted.org/links.htm
MOOCs - Massive Online Open Courses are actual courses, from some highly respected universities, that can be accessed for free. While most don’t offer credit, some may offer credit for a fee.
https://www.usnews.com/higher-education/online-education/slideshows/10-things-to-know-about-moocs-in-online-education?onepage
Crash Course and Study Hall offer video classes on various subjects.
https://m.youtube.com/@studyhall/featured
https://m.youtube.com/@crashcourse/playlists
Feynman lectures on physics:
https://m.youtube.com/results?sp=mAEA&search_query=feynman+lectures