Players get emails/letters/questionnaires typically after someone (coach, friend of coach, scout) sees him play and likes what they saw. Sometimes the contact comes from mailing lists too...perhaps the player was in a high level tournament or a showcase.
The main thing is for your son to get to places where he will be seen by college coaches and evaluated. College camps, HS games, travel ball games, showcases. They have to be seen in order to be known.
Good luck!