Letter of Intent Deadline?

Not an expert on your son's situation, but I'll chime in to get things started.

Please expand on this statement, "...dreams of playing college baseball to help pay for an education". Would he prefer to get a 25% baseball scholarship and still pay $30K/yr at a private school, or have no scholarship and pay $20K at an equivalent (public in-state) school?

DII timelines are generally much later than DI, so the advice he's getting may be more appropriate to DII. Most (not all) DI scholarship money has been spoken for by now.

Depending on the college, those enrollment deadlines may be non-negotiable. A school with a wait list will offer his spot to someone else. If the school is likely to take him late, then that isn't a big issue.

If it were me, I would remove the baseball scholarship from the equation. How badly does he want to play college ball vs attending his preferred college? If baseball is key, then I would start looking for a home today. That might mean DII, DIII, NAIA or JuCo. If baseball is not a top priority, then choose the best fit DI school knowing that making the baseball team without being recruited is going to be a long-shot.

In general,the early bird gets the athletic money before it runs out.  Typically D2s have 9 baseball scholarships to divide among many.  I think your son should make some phone calls (now) to those interested & (presumed) offering coaches if his dream is to play in college and help pay for that education.  I'm somewhat surprised at the advice your son received unless they thought he was going to walk-on.

Good luck.

Since the specific question on this thread is "Letter of Intent Deadline?", I'll answer by saying that this Wednesday is the first day of the spring National Letter of Intent signing period for NCAA Division I and II schools.  A prospect can sign an NLI all the way up until August 1.

However, as has already been pointed out, most of the scholarship money at Division I has already been allocated, and possibly a lot at the Division II level as well. 

If your son's dream is truly of playing college baseball to help pay for his education, and if he isn't getting much interest at this point, it may be time to consider a JUCO where he can work on his game, and possibly get bigger and stronger. I don't want him to panic, but at the same time, he shouldn't just wait until the HS season is over.  He should be actively promoting himself with the best video and intro letter that he has.