DJ75,
If you have a son who aspires to college baseball, please get yourself and him educated about the recruiting process sooner rather than later. If you were to refrain from acting based on your mistaken belief as to when offers are made and the custom of honoring them, your son could miss the boat.
It is true that until an NLI is presented and signed, either side can back out of a handshake deal. It is also true that in baseball, NLI's are written confirmation of deals reached before that time, often quite a bit before that time. There are many ways that players are able to get evaluated, and many ways that players and college coaches can communicate, prior to the NLI. We are seeing more and more players making commitments before their junior year of school even begins.
If you wait until November of your son's senior year to feel committed, you'll find that all the schools' programs have pretty much filled their recruiting classes while you sat on the sidelines.