It sounds like he is acting out more during times when he is bored or he feels he is right. This is very common at this age, once a child reaches the age of 7 or 8 (Boys more so then girls) they tend to start acting more aggressive. They want to show they are men, and therefor know everything.
The main key is dealing with this now.......before he gets into a routine of acting out when he wants to. Sometimes the hardest thing a parent can do is punish your child.....we want to nurture them, teach them and most of all give them the things we did not have or the things we missed out on. So punishment tends to prevent us from following though with those parental desires. The issue is children need discipline far more in todays society then ever before. When I say discipline, I do not mean corporal punishments such as spankings or stiff across the board punishments. I mean that parents must show their children that every action has a reaction......consequences is something that most children know little about. We are parents tend to make excuses for our kids behavior or brush it off. Thus the child has no consequences.
Please understand, I am not blaming you nor am I placing this back on you.......all us parents do it. It is a natural reaction to protect our children. By allowing him to learn that his actions have consequences he will learn when it is appropriate to be aggressive and when it is not.
Since it appears that baseball is a major issue in this routine of his, you may have to use that as part of the consequences. He is old enough for a sit down conversation. Before his next practice or game, sit down with him and talk to him about his recent behavior. Let him know that this behavior is not acceptable and that should he feel the need to refuse to listen, or act out he will be removed from the game or practice completely. I would also suggest talking to his coach. Let his coach know the issues, and let him know what you will be doing about it. Explain that you need his support in the matter and that you will be removing him from the game or practice at the first sign of trouble.
I know this sounds extreame.......but often this type of plan stops the behavior dead in its tracks. The last thing you want is a 15 year old who shows these signs. By taking a firm stance he will learn that there are consequences to his actions and therefor he will be required to follow the rules when participating.
If the behavior is at home as well, this can be done at home to. If you are watching TV and he is being disruptive, he needs to go to the table and sit. How long is up to you....each parent knows what is the best time. While the one minute per year theory works for most children you know your own child far better then I.
When raising our children, we must remember that we are in control and keep that control.......children are very smart and will quickly come to the understanding that there are things they can push. You must decide how far you will let them push then give a gentle push back to remind them that while they are loved they must respect you and follow your rules.
Loving your children is not just about doing for them........sometimes the best parent is the one who refuses to be a friend first.