I would not be too worried. Yes, he is "blessed with size," and projectability should compensate for the less than lightening velo. But "mid to upper 80's" for a 2019 in January is nothing to fret about. I'd say he's more than on track. Not sure if you'd call him a late bloomer, but I believe it was RJM who said, "the college baseball recruiting timeline does not favor late bloomers."
6-6, RHP, 88-90, commanding a FB and at least one quality off-speed pitch next summer at the right events (2-3) should do the trick.