Here's an idea for you to explore-- see if he can earn CLEP credits at a local college/community college. THAT ought to stand in for the middle school recommendation in a big way.
You can also use it as a bargaining chip with your DS-- he has to be self-motivated enough to study for and pass the CLEP exam, or he has to buckle down and just do the work.
FWIW, I disagree that grading on completion is necessarily "nitpicky." I think that has a reasonable place in secondary education, actually, and if the grading scheme is laid out ahead of time, well, students can hardly whine that they didn't know about the consequences of their choices. They may not LIKE those consequences, but hey-- the story of adolescence, yk?
