My own son never went that route, so I cannot speak from first hand experience. My observation was that the PG teams did have deep rosters, in the neighborhood of 18 players/team. Many of them were "pitchers only". As best I could tell, all the pitchers got their shots over the course of the weekend. Other players played more or less equally, which can be good or bad in your mind depending on how you think your son stacks up. Probably an offensive player would play roughly 2/3 of the time. Remember, the guys assigned to handle the teams don't know anybody coming in, so there's no reason to suspect favoritism towards anyone.
I would guess that if the team made the playoff rounds, pitchers would be selected based on their availability (hoping they aren't over used) and on how effective they were before. But in truth, I can't recall seeing a PG team make the playoffs the last two years. It's tough for a team that has never played together to do a whole lot of winning. Some, but not a whole lot. You might want to go to the WWBA site and pull up the Jupiter results from recent years and see how PG teams did. (They are named according to their jersey colors, e.g., PG Green, PG Gold, etc.)
Also don't hesitate to call PG's office to ask how it works. They will tell you straight what you can expect.