johnj, I totally understand your excitement about your son. You are getting great advice here - it is right on target. I just have a couple of questions for you, just to kind of get you thinking. My son was called up to varsity as an 8th grader (small school). It was a great learning experience for him, but it was really, really hard on all of us for a lot of reasons, some of which had absolutely nothing to do with what happened on the field. I wish I could have some of that angst back.
First, are varsity/jv tryouts held on the same day? For our school, everyone tries out at the same time, and then players are divided into JV and Varsity based on the outcome. But it's a small school. If they are on different days, can he not go to both?
Second, why is it so important that your son make the varsity team as a freshman, regardless of whether or not he was held back? If you get too wrought up in this, you will lose time that you could spend just enjoying watching your son play.
Third, if there are 15 seniors and other good players returning, then will your son get much playing time?
If your head coach is any good at all, then what your son does on the field will mean a heck of a lot more than what your son says to the manager. And if the head coach doesn't think he's ready for varsity, and wants him to develop more on JV, it doesn't mean he's not an awesome player, it just means that, for whatever reason, it's not yet his time. If this happens, then set a good example for your son by accepting the coach's decision and cheering on the JV team.
Like dad43 says, these things are often fluid. If he is on JV, there is always a chance to move up to varsity during the season, or at the end of the season during playoffs.
Bottom line, try to relax and have fun. It's not always easy. Mine is a junior, and I'm just now learning to do that!
Good luck to you and to your son!