Hello:
Our son just left home as an incoming first-year at a private, d3 school about 1,000 miles from home, where he will also play ball. So, we went through the d3 process last year - high academic private liberal arts schools.
Here's a basic rundown as I know it, forgive me if this information you already know:
Obviously, there is no athletic money. Merit money will be awarded after acceptance to the school, and it is based upon your son's academic achievement and test scores. You should complete the CSS PROFILE from the College Board during the application process as well. Of course the more selective the school, the higher the bar is set for gaining merit money, so it is really important that you balance that against your son's academic profile if attaining merit money is important.
As an example, your son might be admitted to schools like Wesleyan U, Hamilton, Macalester, Pomona, Whitman, etc. but not be awarded any merit money because of the exceptional academic profile of the incoming class. He might also be admitted to let's say Dickinson, or Willamette out west here, which are also very, very fine schools, and receive substantial merit money because their class profile places your son among the top 25% or so of applicants. So, if merit money is a necessity, you should consider the balance between school selectivity, and where your son's academic achievement and test scores place him on the school's class profile (bottom of middle 50% as an example versus top 20% at another)
Asides from merit money, most schools also offer money to exceptional performers in let's say, music, art, maybe debate, etc.
Then, there is needs-based financial aid, which is calculated from your FAFSA application, from which the school calculates your estimated family contribution to tuition. You can find easy estimator tools on-line that can help you determine what level of needs-based aid you might receive. Household gross income of about $150K and up probably precludes any needs-based aid.
The baseball coach may help your son's admission to the school by sponsoring his application through admissions, but he'll most likely have no ability to influence any form of merit money other than what your son would earn otherwise.
Very best of luck to you and your son.