The best phrase to remember is that "a guaranteed spring roster spot is only as good as the coaches word".
As BOF and others pointed out, there is no NLI or formal/legal document that protects your son's "guaranteed spot" other than the coaches integrity. We have experienced both the good and the bad in that the first coach lived up to his word and my son was not only on the team in the Spring, but also played based on performance. Another kid told us he was promised the same thing, but was thrown off the team in the Fall for discipline reasons. That of course is understandable as is poor academics.
The next school my son transferred to also gave him a guaranteed Spring roster spot(no money), but he fell ill during Fall ball. The coach made it clear that although he could still "make the team if he recovered quickly enough", he was not going to honor the spot we were promised because "winning was too important to him". So while my son still could have been on the Spring roster if he got healthy quickly, he was expected to compete for the spot he had originally been promised. We decided that the writing was on the wall since my son was still ill & 30lbs underweight, so we had him transfer out. The moral of the story is be careful and do your research because anything can happen. For instance the recruiting coordinator who was high on my son to begin with left to take another job. There was no way to know that, so always be polite but ask if there is any likelihood a coach might be leaving. If they indicate it is possible, be very cautious because as others have said, any guarantees go out the window when the coach departs.
This story and worse has played out with others I am aware of as well. They are promised a roster spot only to report to Fall ball with 40 or even 50 kids. The coach then says they meant a guaranteed Fall roster spot(i.e. a tryout as a walk on), nothing more.
So make sure to do your research by talking to other parents/players to find out the reputation and integrity of the coach, as his word is all you really have.
Good luck & keep us posted.