Quickie

Quickie, I believe that the "good" pitching coaches can spot it instantly. Notice I had to qualify my answer on the coach. I've seen just as horrible pitching coaches as good ones. Some wear the title but haven't a clue.

Let me be the first to say I am NOT a pitching coach but I bet he'll work on legs and core to get a pitcher consistently in the upper 80's...IMO.

Sounds like the 88 may be his max velocity right now, 84-86 his cruising speed. Consistency is there, that just may be the most he can get out of his arm.
What has your son worked on to develop his arm strength?
It may just be that he has reached his peak velocity, or perhaps his mechanics need tweeking, not knowing your pitcher, his coach, it's very hard to answer that question.