He wouldn’t be behind but the natural progression for a kid who took calc BC as a sophomore is some kind of higher math jr and sr year (AP stats only requires Algebra2 so it wouldn’t count as higher math, although stats is a very useful course, one I personally would prioritize over calc for most students…)
You’re right though, if he’s bumping into high school limitations, it won’t be held against him in college admissions. However he might want classes that match his background and aptitude, hence the suggestion of dual enrollment classes.
Dual enrollment at a CC= AP though, just faster paced. For math, should be considered as a “taster course”.