Advice for child struggling

This may be long, so I apologise now, but I don't know where else to turn, I guess this may be more of a rant? I don't know

I am a mum to 4 children, G12, B9, b8, g7.  
G7 is lovely, a typical cheeky, occasionally misbehaving for her age,
B8 is a sarcastic emotional little sod, 
G11 nearly 12, is far too grown up, but is headstrong and doing OK

Its the B9 that I wish to ask about, is anyone else going through this, does anyone understand I have no idea what I am asking, 

he has struggled for a long time, behaviourally and with his temper, he has always been very quiet in terms of personality, as in doesn't seem to have developed his own, and mirrors the wants of others around him (ie presents etc)
He is very very quick to temper, very quick to smash things, attack his siblings, and just generally hard work, 
His siblings, when he kicks off are so scared of him, and will run and hide. 

He non stop asks question after question, getting more and more wound up, often resulting in him losing his temper, when he talks he talks with a whine and a cry, he has learnt to say sorry but in a sarcastic way and carries on with what ever he was apparently sorry for!

If he's running and you would rather he stop (say for safety if we are out) he will say OK, then continue to run, so we ask him again, and he gets angry, you explain why you want him to stop and he won't have it
When shopping, he is into everything and doesn't seem to understand personal space etc, don't take him shopping? He is pretty much excluded from school currently, as he a) keeps escaping,  b) throws chairs c) attacks teachers, and so forth
he has not been in main stream for a long time now, and the provision he is currently in cannot cope with him, 

When I say about questions, a couple of examples, 
Whys that car red?  because the people who brought the car wanted it red Why did they want it red?  maybe its their fav color  
But why?  why do they like red?  
By this time hes most likely to be breaking down

the other result, is he will ask us a simple question, *is it tea time yet*  Yes it is,  he will repeat it in a different way, we answer the same again, he will then start screaming we are not listening to him, that's not what he asked and so on, 
If he still isn't happy with anything we say, he will attack a sibling that's closet, if none close, he will run to his bedroom screaming horrid abuse, slamming doors (recently smashed his tablet) if his brother is in the room he will scream in his face or hit him until he leaves. 

this is just the tip of the iceburg as the saying goes.  

we are lucky to be moving to a 4 bed house, so he will be able to have his own safe space, but I know this won't solve the issues. we do have social involved, who wanted CAMHS (?) involved, but a) their waiting list is over 18 months, b) they won't even look at the boy, as they claim the reason he is like this is because of home circumstances, 2 yrs ago I had a mental break down, and my mental health has been wobbley since, yet we had issues way before then, including home support n the 8 months leading up to my breakdown for his behaviour, yet that isn't good enough, and they state he is only this way because home life is (Removed, profanity), (we are moving due to safe guarding issues) 

I guess I don't fully get the point of writing this bar sounding off, its causing friction between my husband and I, and I sometimes just want to walk away, 
his siblings live on eggshells. as often just looking at him on pure glance will set him off that they are apparently staring at him and he doesn't like it, when its simply not the case, 

its just so damned hard, and I don't know what to do any more, my husband is often in tears because of the words that spills from the boys mouth.