I would echo what CD said and let my play do the talking. I think all coaches knows that every kid trying out for baseball at that school wants to be a varsity player. They are the ones that get the chicks, right? Or maybe thats the long ball guys?
I want to add one more to CD's list. Between innings, be the first guy to the dugout or to my position. ALWAYS RUNNING! Hustle doesn't have an off day in baseball.
The scenario you posted sounds like a tough task for this kid as a freshman. 15 returning seniors? And then throw in some juniors and a couple good sophs. I'm not saying it can't be done, but be realistic about the situation and if your son doesnt get placed on the varsity have him use it as motivation. How good is the HS he is going to? Were they conference champs last year or were they non-competitive? Having a first year varsity coach might bode well for him if the coach wants to weed out the non-producing seniors. Don't worry, you can't control it.
Before your son or you ever say ANYTHING to the coach other than "Hi, how ya doing?" You better take a step back take a deep breath and count to ten. Wait another minute and repeat the process as many times as necessary. You get my drift?