I agree with all said here. Most coaches want to win. Last year on our team, the three starting pitchers were a freshman and two sophmores. There were 2 senior pitchers and a junior on the team. I think when we started the season, the parents of one of the seniors was very disgruntled. However, over the course of the season, he saw how his son performed and how the younger guys did. He wasn't real happy, but he quit grumbling so much.
There will always be some politics. Seniors will usually get preferential treatment, unless an underclassman is head and shoulders above him, but generally the coach wants to win and will play those with the talent to help him do that.
If your son is that good, I wouldn't worry too much. The other thing I wanted to say is, I wouldn't try talking to the coach and promoting him too much. Maybe let him know that he is there so the coach knows generally who he is before tryouts begin, but don't pump him up too much. Coach probably hears how good a parents kid is ad nauseum. And he hears it from parents whose kids really are good as well as from those whose kids are very bad. He has no way to know until the kids hit the field. If your kid has the stuff, he will play. Be confident in that.