CoachVictory,
As others have posted, you partially answered your own question when you mentioned "so many variables". But you are also being a smart parent by starting to help explore what level your son might be able to play at in college.
I wanted to make a comment about D3 baseball. My son is starting his junior year at a small D3 in Minnesota, and his conference demonstrates the huge variations in level of competitiveness among D3 programs.
At the bottom of their conference are 1 or 2 teams each year that his old HS team would have a good chance of beating. Almost any player who had won a starting spot for their HS team would likely make it on the roster at those schools, and would have a good chance of being a starter as a college freshman.
At the top of their conference there usually are several strong teams including one that has repeatedly been ranked as high as 4th or 5th in the nation among all D3's, and has a history of players being drafted. Obviously much tougher to make the roster there, and of course to earn playing time.
Even at my son's D3 which is about 3rd in this conference, typically every player who earns playing time was the "star" on his own HS team. My son was the ace of his HS pitching staff and earned all-conference and higher honors, but has found it a huge step up to compete at the college level. He earned only about 10 innings freshman year and a little more than 20 innings soph year, and has worked his butt off to try to earn a bigger role this year (junior). There are approximately 50 players competing for a spot on his team this fall - over 20 new players including JUCO transfers and freshmen. He is still very happy with his school and the baseball program, but it has been at times a surprising journey.
My point is, I agree that most HS starters could find a spot on SOME D3 roster, but D3's vary tremendously in the competition level. If your son decides that playing college baseball is extremely important to him, it will help a lot to do your research, as you obviously have started to do.
Best wishes to your son, and CONGRATS on his great performance off the field, in the classroom!
Julie