Happy update: not only is he managing a full-time academic load this semester, he’s doing very well. In fact, his Calculus professor recently recommended him for a NASA internship and several of his classmates have asked if they can hire him as their private tutor (he declines, saying he will help them for free).
My takeaway as a parent? Patience is a virtue! 
Regarding the notion that unschooling is tantamount to enabling (and encourages an avoidance of hard stuff), well…he logged 254 hours in the school’s tutoring center last semester. He works harder than anyone I know, and his professors have taken note, too. So, looks like unschooling worked out pretty well in his case.