Summer before 10th grade: 81
Summer before 11th grade: 86
Summer before 12th grade: 92
These were maximum effort velocities. I expect by next spring (the start of his first collegiate season) he will be sitting comfortably at 87-90 and perhaps occasionally hitting a high of 92+.
I attribute these numbers and the increases year over year to (1) physical growth; (2) coaching and refined mechanics; (3) natural ability; and, arguably, (4) lots of long toss; and (5) lots of innings pitched versus good competition.