I would think those numbers are pretty typical for a kid who is working on his throwing. I would guess my son's numbers were about the same.
My son is a 2015
January 2014.....up to 84 at an indoor showcase
April 2014....83-84 in his first HS start of the season
May 2014.....85-87....and 86 in the last inning of a 7-inning game
July 2014.....sat 85-86....up to 87 a couple times during travel ball
April 2015....first game of this spring 86-89....hit 90 4 times, 91 once, though I question this guy's gun a bit...it's always seemed a little high
Since....has sat 87-88 in last 4 or 5 games...hitting 89 often. Last 2 games, he hit 89 on the last batter of a CG
Last spring he was 5'8, maybe 145.....he's now 6'0, 170....just turned 18 two weeks ago, so he's 9-10 months younger than a lot of 2015's