A happy bit of follow-up:
I was griping to a good friend (local) about the co-op and how DS7 really liked the concept of the class, if not the execution. She's homeschooling her 9yo HG (my best guess) DS, and we got together at least weekly last year for social time. We needed it as much as the boys did!
She sympathized with me about the situation, and then she said, "Why don't we do some math and science stuff together? The boys may not be at exactly the same place, but we work on a spiral curriculum anyway, so it's okay if they're at different places on the spiral. And then afterward the boys can play together."
Heaven! She's a dear friend, DS7 loves her son and shares similar interests, and they'll have a great time working together. She and I have similar approaches, too, so I'm not worried about the teaching issue. We're a good fit. Really, it's genius!
So it's not a full-fledged co-op, but it's a very good set-up with a very good friend. I'm thrilled!
Sometimes life has a way of working out...