If this is a new behavior, there most likely is a trigger behind it. Behavior is purposeful and there could be different reasons for this such as- being bullied, displacing anger onto others, struggling academically and lashing out onto others, hormonal/mood changes, peer pressure/clicks, drugs, etc.
He is old enough as to where you may want to sit down with him and point out to him that this is not the norm for him to act this way. Let him know that you're confused as to what may be going on and that you'd like him to help you understand this situation better. Ask him open ended questions (what would he like to talk about/what is on his mind) Note to him that even if what he's dealing with may not be something you can fix for him, you'd assist him in finding ways for him to manage it.
Explain to him that it is best to deal with problems early on before they become unmanageable such as the possibility of being expelled. You can encourage him to talk to you or another trusted family member. Even offer to him to speak to his school counselor if for some reason he feels uncomfortable speaking with you.
Try to show him that he only has control over himself and not of what others do or say. As a result, he can redirect this anger/frustration into positive things that would benefit him i.e. his school work, sports, friendships, etc.
Yes, you're correct- something is causing this change in behavior. If there is another family member he would be more open to speaking with, that could be another thing to try. You can also let him know that nothing is too shocking or embarrassing that you as a parent won't understand. He may also fear punishment if he feels that he had caused something to happen and you could let him know that you'd rather have him be honest with you so you can assist him.
You can ask him if he feels too uncomfortable that he can choose to share just some of the facts so you can get a better idea of what is going on. Let him know that he can keep some of the details out if it's too difficult to speak about certain things.
Ask him what could he do differently if he feels provoked in the future rather than fight? And, he probably cares but is saying that he does not because he feels guilty and a sense of failure. Continue to be supportive and remind him that his behavior is a reflection of his inner self and that others may be wanting him to loose control (depending on what is going on) If you could speak to any of his teachers to see if they've noticed something different and unusual lately is another thing to explore.
You could say to his girlfriend something like "I don't understand what is going on with him lately and I wish I knew..." She then may offer to share something with you. If she's quiet, you may ask her if she's noticed anything different about him and if the relationship is going well. You have the right to worry since it's uncharacteristic for him to behave like that.
The teachers may have observed something. Perhaps start with those teachers where his grades are the lowest and move on to speaking with the teachers where he's doing well in the classes.
You can always contact me again via this site. Write for Dr.Rossi at the start of your post to do so.
I would suggest that if your work has EAP (employee assistance benefits) or you've got insurance, to use them for the therapy. EAP should provide some free counseling sessions and the insurance will give you in network providers who work with teens.