I realize this is a baseball site but I have to open up the academic discussion since Fenway tried but it was overlooked.
While playing baseball should be a consideration it should not be the primary consideration. What does your son want to be when he grows up if the baseball thing doesn't work out? Someone who wants to be a teacher may go to a very different school than someone who wants to be an engineer. I know most of our kids don't want to consider that baseball will end for them someday, but it is something that will happen and he needs to really think about what he wants to do after baseball.
After you lock down a sort of career path ask what type of school does he want to go to? There are small schools and big schools. Inner city schools where each building could be 5 miles apart or schools set in a farm community where everything is within walking distance. Within an hour of home, or is he okay going cross country? What can you afford to pay or get loans for if there is no baseball money?
Answering THOSE questions can narrow down the type of school he wants to go to, then from there you can check out their baseball programs. I don't think a 2018 is too early to start to make a list of schools. Once you have your list you can follow the timeline on the home page of sending out letters/videos/summer schedules to interested coaches and maybe find some showcases where his target schools will be in attendance.