@Mom2aphysicsgeek, thought I explained it somewhere but basically it comes down to depth over speed. He’s always been an in-depth, laser-focused learner who feels he has to know everything about a topic before moving on.
Well, that and the practical issue: this was his first ever school experience and we figured a part-time pace would allow him time to adjust to the game of school. He’s registered for 11 units next semester, so he’s closing in on that 12-unit full-time pace! 
Plus, there’s some LD issues.