As you have these discussions, try not to be immediately dismissive of the coach who seems to hedge some. Often, he's not trying to be disingenuous; rather, he's trying to respond pragmatically to a situation involving a lot of moving parts (many of which are completely out of his control).
Instead, listen carefully to the overall situation he describes along with what he is offering specifically to your son. As you listen, ask yourself if what he's offering makes sense, given the broader context he's describing.
For example, he may say something like,"Based upon what I know today, we're able to offer your son at least 40% of his total costs. I'd like to be in a position to offer something more like 50%, and I may find myself able to do that; but, it depends in part upon which of my rising juniors signs a pro contract (and/or which of this year's recruits sign pro contracts out of high school)."
This is not a professional team trying to apply a "salary cap" that can be exceeded if the club is willing to pay a tax for exceeding the cap. It's a hard-and-fast, zero-sum game. Every dollar he gives your son takes a dollar away from another player on the current roster or recruit; and, he has to be exceedingly aware of that. This is especially true when considering a situation involving a player who won't matriculate for another two years.
Meanwhile, the roster of current players and recruits is as fluid as it can possibly be. For this reason, his hedging may well be both a response to that reality and a good faith effort on his part to commit what he knows he can commit today with an earnest desire to find more money for your son as roster and recruiting developments allow.
If this is the sort of thing you hear and he seems genuine in what he says, find the opportunity to come behind his comments and see if current or recent players have (1) heard the same sort of thing as recruits and (2) found, once there, that he lived up to his professed intentions.
Best of luck to your son!