This is a consequence of the NCAA's institution of the 25% minimum rule a few years back.
Previously, a school could lock down a player for as little as "book money". The NCAA didn't like players being taken off the market for small amounts, and stated that it thought the rule change was for players' benefit. But the problem is, if you don't command 25%, you end up with nothing. Even with "book money", you would be a "counter", i.e., someone the NCAA would count towards the program's maximum roster size of 35. When cutting you wouldn't help the program get under the ceiling, your odds of being on the spring roster were pretty close to 100%, excepting only if the coaches concluded during the fall that you were a problem kid.
But when you have zero money, now you are not automatically a "counter", so you can be cut. And no matter what someone promises you orally, things can change. Some people don't keep their word. Sometimes coaches are fired and replaced. Some times a guy meant to keep his word but later finds himself in a predicament he didn't anticipate and his only out is to cut you.
So, there are risks. The best bet is to post a request for information -- ask people here to send you private messages about experiences at the school and whether this coach has a track record that engenders trust, or whether perhaps others have been led down the primrose path this way.
There is one other factor. Suppose this is a school where your son could not pass admissions on his own, but he could with the coach's help. That may be enough to sway your decision. That uses baseball to get an opportunity you wouldn't otherwise have had. Even if he does get cut from the team (and there's no saying he will be, as pretty much every team has 8 or more walk-ons), that wouldn't result in him losing his place as a student. For many, this is enough to seal a deal. Whether it is enough for your son, only you and he can decide.