Could not let this go unanswered, what a difficult situaton for you . As a teacher in a PRU (Pupil Referral Unit - for excluded kids), I have seen this type of behaviour often (and actually get it myself to a degree).
I would suspect one of the reasons is because he does not know WHY he is angry. And also perhaps does not know how to stop it.
IMVHO he DOES need counselling - he probably would deny it is about his dad and being left etc but it prob is. He is probably also really guilty about you too now and doesn't want you to be sad or anxious (lovely boy that he is ), and so really needs to talk about it with somebody else.
What does he say about it (when not in 'rage' mode?). You do need to be talking (in matter-of-fact way) about what he thinks he could do to cause least desctruction etc. He really needs to know from you that IT IS OK TO BE ANGRY but it's what he does with that anger that's important. Don't underestimate eating often too (even a Mars Bar) as hunger REALLY exacerbates this type of irrational rage (does in me too ).
There is a great kids book called 'Mighty Fizz Chilla' by Philip Ridley which might resonate.
Think it's ridiculous they haven't referred him already. Think this type of 'issue' (yuk, sorry) can really be dealt with early if identified. He may always feel the same way but learn strategies to deal with it - it would be of great benefit to him to learn those strategies now and not have to as an adult (with lots of expensive therapy).
In terms of hope, we see many, many children reintegrated back into mainstream education having learnt effective strategies for anger management, and receiving appropriate counselling/support. Parental support and love cannot be underestimated here and you are clearly showing it in bucketloads, so am sure there is no reason your son can't learn how to deal with it and be successful too.
Best wishes and good luck.