Meet with his guidance counselor; get him/her to give a tutorial on the process, where he/she sees your son so far (hard in a large public school where they might not have even met, easy in a small private school), where kids from the HS have gone (we had a program called, I believe, Naviance, which - through graphics - painted a picture of what academics a kid needed to get into various schools).
This will create a box of schools based upon academics for non-athletes; most, if not all D1s have some latitude in lower standards for athletes (some more than others). Also, it will give you the knowledge of any adjustments in his HS curriculum which may be needed for certain schools (e.g., "most rigorous curriculum.")
Meet with his teachers to get a feel of what they believe is his academic potential as well as flushing out any academic/LD issues.
If money is a limiting factor, try to figure that out now so the family is on the same page; d3s without academic and athletic scholarships can run 65k+. Also, if you're self-employed, you may be able to do some serious financial planning which could favorably impact any financial aid offered - here, you need to hurry up and figure that out.
There used to be a website - both terrifying and intimidating - on the pure non-athlete college selection process: college confidential; if it is still up there is lots of good (and bad) info.
Its alot of info to uncover and can appear daunting; one step up the mountain at a time.