Jones fan,
I don't think there are magic words. It is a process over a course of time. The more consistent the process, the more your son is probably wanted.
Some items to consider:
1.) has the coach seen your son play and what is the feedback?
2.) has the coach invited your son for a visit, to meet with the coaches, meet the team, see the school, stay on campus, etc?
3.) does the coach call regularly? It may not be every week, but it is regular.
4.) does he tell your son what he sees in him, where he sees him contributing in the program?
5.) does he emphasize with your son, and you, the academics the school has to offer?
6.) does he talk with your son about Summer ball what he sees can happen for your son?
7.) does he talk about a plan for your son to progress within the program, while providing guidance on the amount of effort and improvement/production on the field that will be expected from your son?
8.) does he sound like a coach who wants to coach your son? Does he actually say that?
9.) is the coach and his staff doing everything they can to facilitate your son applying and do they talk about supporting his application to the extent that the school permits such support?