I would say from what I have seen through the years.... 14-15 yr old velo is not crazy important. Honestly , 2018 is not the norm.... he has a gifted arm. He should be mid 90's as a sr... I would say D1 , outside chance draft.... bu.t still too early to know anything
BUT
My 2013 was 77 as an 8th grader , 81 as a freshman, 83 as a soph , 84 as a sr.... 85 as a freshman (juco) 87-88 as a soph (juco) has mad movement , location, and work ethic.... he's going to Tennessee Tech next season..... years of hard work, and people telling him he was not good enough...
My point is you do not know yet.... too many factors.... BTW 2013 is 2018's pitching coach 
Velo is important.... it gets the scouts to show up.... but 0's on the scoreboard keep you on the mound.
Heaving had 4 sons at this age, and they are all different... put your son in the best league you can where he can succeed... work on all aspects of pitching, play to your strengths... work on the weakness...
Looks like the mph after freshman year were a grind. Did he stop growing early? Was he on a pitcher specific workout? Really happy for him. And you must be proud how hard he worked to grind out those needed mph.