Ultimately those who perform play. But a non scholarship player might be the last one to get that opportunity if at all. I say might because there can be other circumstances. It's a nothing to lose situation for the coach. He's not giving up anything. About 20-22 players play enough to impact the success of the team. If your son is number 28-35 (non scholarship) where do you feel he stands in the mix? That you're asking here means you're not sure and the coaching staff hasn't given your son a definitive answer. I believe if they really wanted him they would be offering money or giving a solid reason why they're not yet but they really want him. Your son should ask who are the non scholarship players who are starting. A player is never guaranteed making the team from year to year.
Once your son steps on the campus the five year clock starts. Once he steps on the practice field with the team he sits out a year if he transfers. Not playing and transferring could mean sitting for two years. Thank the NCAA, who looks out for the best interest of players (cough, gag) for that rule.