JDad, first of all, congrats on your son's offer!
Last month Midlo Dad posted some excellent advice for a recruit trying to see if his top choice school was interested in him. I believe the following email would fit your situation as well:
E-mail: "Dear Coach _____,
"As you know I have long been very interested in your school and your baseball program. I have appreciated your taking the time to evaluate me and to consider me.
"Recently, I received a substantial offer from another school's baseball team. They have asked that I respond to them by _____.
"I would greatly prefer to come to your school and come to your program. However, if you believe that you are not heading in that direction, I would need to take advantage of my next best opportunity.
"If you could let me know candidly whether you foresee any opportunity for me with your program, I would greatly appreciate it. Thank you again for your consideration and for your help with this difficult decision."