I think your best option might be to find a used college text and exclude any chapters/problems that are above his math level. There is extremely little true astronomy available for high school or younger, that I've found. Gravitas Publications will have one eventually that is a legitimate preparation for college astronomy.
I have not been impressed by AOP science (one of the options mentioned in the blog). I have not seen their astronomy course, though.
IME astronomy labs are almost entirely math and computer simulations. Just mentioning in case your student isn't expecting that. I did find these free lab resources.
https://www.oercommons.org/authoring...es-for-as/view