Policies such as that tend to be set by each state's governing body for interscholastic athletics. A quick search led me to the "Handbook"for the Florida High School Athletic Association.
Section 9.5.1 (p. 61) states: "A student has four consecutive calendar years of eligibility from the date he/she begins ninth grade for the first time."
Later, the "Handbook" adds that a student can't be older than 19 years, 9 months to participate.
I'd invite you to check out the following link to see if a distinction is made between junior varsity or varsity sports. If you see none, I'd recommend that your son ask his high school coach.
Florida High School Athletic Association "Handbook" Best of luck to your son!