Yes, check with the school too and see what they say. Maybe he is just feeling left out, brother and sister getting more atten. As middle children, that can happen and also sometimes other siblings have more going on.
Sounds like a lot of frustration going on. Sometimes the middle child can play the others against eachother, and sometimes they gang up on him. Kids get angry, its how you handle the anger that trains them how to deal with it.
Also you should talk with his teacher. If he does not have any of these problems at school, then you know its home related (siblings, how he has trained others to respond to him, etc). If it also is happening at school, then its a much bigger problem. But I would have to know what is going on at school in that case.