Does he get on well with your 14 year old son, are they good friends, Could he have learned or be copying any of this behavior,are you sure that there are no other factors going on that upset him .
Definitely take him to get evaluated. How old was he when you got custody of him? Does he remember what happened during that time? You did a really good thing by stepping up to the plate to care for these 2! Do they have contact with either of their birth parents? That might cause confusion and conflicting emotions. Any therapy he gets will do nothing except help him.
He needs to learn how to express his emotions in a better way than through physical violence. Can you take him swimming or get him involved in sports? What are his interests? Does he like music or drawing? Try to encourage these parts of his personality. Keep him busy. It's never a good thing to have a bored child. How is he doing at school?
I agree to have him evaluated. From his history, it sounds like he may have an attachment disorder.
There is another tool you could try, "video feedback". It's really amazing, some people have no idea what they look like and how they're acting relative to others in the room.
You get a video camera and put it in a gathering place in your home - or get two, one for the living room, one for where you eat meals. Announce to the family that there seems to be a lot of fighting and bad feeling in the family, and we're going to video so we can all look at it later and see if we can all figure out what's going on. Each person will get to evaluate THEMSELVES ONLY, and we can try to come up with better ways to get along, to make our home a happy one.
Then turn 'em on and start taping. Go through the tapes yourself to mark places that are fractuous - not ONLY him, but add in some others too, and then have a family viewing night with popcorn. Each person watches, and ranks themselves (what could they have done better in this situation? Anything? What set it off, any ideas?)
And then get out a posterboard to write down ideas. "maybe we should rearrange the seating at dinner", "maybe we should have a no touch rule", "maybe if we start to fight we should be allowed to leave and come back to eat when things are calmer". Etc. They often come up with very useless things, but still write them down if they are positive, or they are trying.
Most kids do remarkably well with this. The trick here, is to make the tone neutral, fun and positive - and like you're trying to investigate to find answers - do not let it turn into a session of "we can't stand to be around you so sit right here and watch what a jerk you are".
Best wishes. From experience with adults and children, this is a tool that REALLY works.
I think that you should take this boy for testing. Especially if his counsellor is already saying that he is o.d.d. But I would question if she/he has the appropriate medical degree to be diagnosing anyone with anything. I would be asking for a second opinion. It won't hurt to get him tested, in fact, it can only help you and your family situation. This way the boy gets the help needed and so do all of the family if need be. Do these two children see their father or mother at all? Have they been able to talk about how they feel about their situation? Are they into any extra cirricular activities? Children need to be active in order to gain better control of their bodies and listening to authority better. Start with going to the family doctor and getting him referred to a child psychologist or achild psychiatrist. I think the first one is the one you need. They will be able to test them by having you and the boys teacher fill out paper work in order to find out whether or not he is truely o.d.d. or any other form of a.d.d./a.d.h.d. or anything else that could be possibly contributing to his behaviour. If you ever need to talk or vent, please do not hesitate to send me a private message. Take care.