High school is a big wake-up call for a lot of kids. They are expected to be able to handle their work & assignments without constant reminders from teachers (and from parents). They often have a new set of friends and classmates, either from several middle schools coming together or from new groupings based on class "teams" and choices.
They also find that they are trying out for teams with a limited number of slots, particularly for freshmen. I am not sure what the custom is in your district - sometimes parents are expected to show up at tryouts as one poster mentioned below, but at others (like mine), helicopter parents are discouraged and looked at warily. Coming to games and meets is one thing, but attending tryouts would not be valued by coaches, and it might be seen as overkill. If your district is like the one Mum4Ever describes, I'd be shocked - but it's possible. But going to the coach would be seen in most districts as interference. Your belief that your child was "cheated" out of a spot on the team may show a blind spot about your son's ability (what coach wouldn't want such a talented athlete???) or it might be perceived as a challenge to the coach's ability (never a good idea). Isn't it possible that the other kids selected were either more talented or a better balance with the existing talent, especially in a sport where players have different skills for different positions? A team doesn't need 4 left fielders if they are short on pitchers.
And thousands of kids come from homes where both parents work, and team acceptance is not in any way based on whose parents show up for every tryout. That would be a horrible way to choose a team. (And we have at least one example of overzealous parents who have actually been banned from games and practices!)
Instead, work with your son on having only one plan - baseball - as being unrealistic. He's a freshman, and he's tried out against sophomores, juniors and seniors. It's a limited size team. So he could show his interest by volunteering to be an assistant manager or equipment manager (not glamorous, but that's the point).
He could also work to get in good physical condition in some other way. For example, in our high school, the track & field team is open to EVERYONE with no tryouts at all. Freshmen, in particular, are encouraged to come and experiment with different events, and to participate in training which builds muscle, coordination and endurance. The head coach doesn't even like for new kids to feel pressure about what to compete in - they should just work with different assistant coaches and get exposed to the many possibilities. Not everyone has to run fast - the shot put and discus athletes aren't fast at all. But the smaller kids (frequent in pubescent kids) may do very well with sprints or other events despite not having a lot of upper body strength. Moreover, a ton of athletes for other sports in other seasons use spring track/field or fall cross country as ways to get in and stay in shape. SO these sports are full of soccer players, tennis players and baseball players in the off season. Participation gives them a chance to be reviewed by coaches who may communicate their impressions to other coaches.
As for the chores and nagging - stop. The nagging isn't working. Having a written list might be just right, as suggested below. Choose, and prioritize, the chores you want done and the timetable. There have to be repercussions/consequences too - kids always want something, whether it's a ride to the mall or an allowance or a cell phone or time to play video games. Tie those to what he has earned by being a responsible family member.
Also work on your expectations for his grades - I'd suggest that his effort is more important than just the letter grade - so if he's lethargic and non-participatory in class, that's a red flag. Set up a teacher conference and discuss where he's falling behind and what he needs to do - after school help is free but up to each student to seek out.
I think every kid needs to learn how to deal with disappointment. Is it possible your child has had everything work out for him (acceptance on every team since he was little, for example)? Perhaps his grades have come easily for him? Perhaps he hasn't had to develop a sense of responsibility because you always keep after him to get work done? It's time to upgrade your approach based on his age and maturity. I know you'll always think of him as your "baby boy" but he's not - he's turning into an adult. That means he has to learn that things don't always work out perfectly and they aren't easy. But one disappointment shouldn't send someone into a tailspin if he has other areas in which to succeed and excel. If this is not something you can turn around on your own, getting some good family counseling with someone skilled with adolescents would be a great short-term solution to get you all on the same page.