Lots of choices. Maybe have your dad join the Yahoo Group HS2Coll to get ideas for online courses. You don’t have to do one “school”; you can piecemeal courses. Here are a few:
Coursera
MIT EdX
PaHomeschoolers
Online G3
The Potter’s School
Lone Pine Classical School
eIMACS
AoPS
Stanford OHS
Lots more to choose from. My sons have all taken courses from various online schools including AoPS, Coursera, OnlineG3, The Potter’s School, University of Missouri, etc. We did more at the local community college and homeschool co-op, but the online courses were good for the most part.
You need to figure out the homeschool laws in your home state and abide by them, as well.