mik,
I applaud you for starting to research this now. It is really, really tough to generalize with D3s. I think you'll find quite a few West coasters on D1 rosters because they recruit nationally. With D1 public (state) schools you'll see an abundance of players from that state (naturally). D1 private schools have more options with their rosters. Very few D3s recruit nationally either due to academics, athletics or both. My son's team and conference (D1 East Coast) had about 50-66% of their players west of the Mississippi river depending on the year. As you research this more and more, you'll realize how all of this works. The best thing to do IMHO is figure out what it is your son wants (major, city or suburb, big school, small school, etc...) then figure out where these schools are. As you get farther and farther down the recruiting path you'll gravitate toward where your son fits.
In terms of raw numbers there are vastly more D3 schools on the East coast. There have been a number of West coast posters over the years that complained about that and came to that conclusion.
Good luck.