Algebra I is a prerequisite for geometry. Algebra I can in some cases be stretched to a two-year course. Then comes a year of geometry, but somehow he had two years of it. You're already up to four years. Next he takes Algebra II, which is only a one year course, but I guess he took it twice. He's up to six years. Then comes a year of trig (or pre-calculus), followed by a year of calculus.
If someone is sufficiently good at math, it's entirely possible he was allowed to take the more advanced courses early, possibly simultaneous with taking the normal ones for bookkeeping purposes. (After all, they'd be easy by comparison.)
Also, I took geometry in 6th grade. That isn't necessarily a high school course.