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.