JAC: Its a good first start that your son received a letter, howver, do not read anymore into thah that. What is important, is the school knows who your son is. DI baseball coaches can make a telephone call to a prospect beginning March of the players junior year or the coach can a call once/week in the players senior year. At this pont, the coach has no idea of your academic status (GPA,ACT score) and will make any offer(in most cases)without knowing so.
Telephone calls from coaches, handwritten notes, invites to the college, visit from the coach in person, request for video are serious steps in the recruiting process.
Send the coach a thank you letter for his interest(needs to be signed by your son not the parent)along with a copy of a future high school schedule along information about your grades. If you have a player profile, send him one.
This letter is probably just the beginning, do not read into it that the college is actively recruiting you son.
Many colleges have prospective student-athelete questionnaires that can be completed at the school website under athletics. Check this out as well.
Good luck