I hear you on the poor quality of community college classes! This is why I think that DE classes are mostly not worth it, unless they’re AP classes taught in the high school with the teachers certified to be teaching the same curriculum as the flagship state U’s corresponding classes.
Is your local public high school any good? If it is, the best option for him would be to enroll there, taking all AP classes. This would provide him with a typical social environment, and classes that likely have high standards. But I guess that if this had been an appropriate option, he would have already been there since Sept '21, when school was pretty much back to normal.
Many private and public 4 yr colleges will allow a senior to take one class a semester for free. I suggest that you approach all the 4 yr colleges within driving distance of you. He can probably cobble together 4 or even 5 classes a semester this way. However, they won’t provide him with a normal high school social environment, just a college in-person classroom setting. Maybe he could also join some clubs at the colleges, fun social clubs, like the ping pong club or the gaming club, just to get some relaxed hang-out time with peers? After all, he’ll only be one year younger than the freshmen.