Lori H,
Is there a community college or University within driving distance? Perhaps auditing a class or two would be a good addition to online classes? You are still the homeschooling parent, and you grant the grades, but the college classroom provides the course content.
It's so hard to find a balance between listening to other people enthusiastically blab about their experiences (the girl and teacher at the picnic) and wanting what they seem to have, and, on the other hand, recognizing that the world is a wide, wide place and what works for one doesn't work for all.
How much to lead? When to follow? Such difficult questions! I keep myself for overanalyse-ing by asking myself 'given all the constraints - what is the 'least worst option' at this time?'
If I was in your shoes, and I really loved that co-op class, I would consider carefully 'making a deal' with my kid that he do the first 3 examples from the grammar worksheet and if they are correct to leave the rest blank. The grade comes from you ultimately, and you are the one who decides what he actually needs. If my son were, say, 8, and getting a lot out of the writting part of the class, I would do the worksheets myself. It's trickier with a teen and may not be worth you while to go that far.
Have you asked the teacher if she would act as a writing tutor with him individually, so that he can get a more individualized experience? Is there anyone else that would be willing to act as a writing tutor?
Maybe your son would be interested in developing an online website that teaches other kids 'how to be a better writer' and he would be willing to collect all that is know about teaching writing? Lots of kids are entering college without solid writing skills, and interviewing Professors who teach Freshman Comp would be a great way to learn more about writing, while practicing writing, while providing a needed service.
Love and More Love,
Grinity