No, I would not recommend that a recruiting letter come from a parent.
IMO, a typed letter is fine, so if you want to help get him started by ghost writing a letter or two, I think its OK as long as you keep son very involved in "his" recruiting process and he reviews, approves and signs all letters. It would be pretty embarrassing if he got a follow-up call from a coach and had no knowledge of the letter sent or its contents.
Keep the intro letter short and personal. Focus on letting them know of your son's interest, position, and where (tournaments/showcases etc.) they can see him play this summer. Attach a standard profile (samples under recruiting link) with his contact information, coaches reference info, & baseball stats.
Reminder: Include your son's e-mail address with the contact information. We found that many coaches today like having the ability to jot a line or two to a prospect vs. a call or using US mail. ALSO, this maybe the time for your son to get a new e-mail address.
I've seen many kids who have inappropriate sayings or abbreviations in their e-mail name. It may not be the impression you want to give a prospective coach!