Hi! There are several online options that are inexpensive if you don’t mind staying with the same curriculum they use in public schools. If that is why you pulled him out and want to use something other than what the state offers, then text books are probably less expensive than the online courses. That said, we are using online history for our oldest this year and I really like it for high school. I still check her work and can adjust grades if necessary but it takes a lot of the work load off of me.
As far as getting him used to it, I would probably sit down with him and come up with something that will be realistic for him. Probably write or print it out so he can have it somewhere to refer to just to keep him on track. And then aside from that it’s just going to take consistency and time to get used to a new schedule.