My eldest has Autism and with it some very challenging behaviours. Including big melt downs, lashing out, threatening to kill and most recently he's been pushing back beyond what I can restrain (he's 9).
We have trialed different coping strategies over the years until we hit upon ones which work. There are two we use that spring to mind that could be worth trialing if you feel it would be appropriate. The first is at school pick up all the frustration is ready to bubble out so its quick as possible to car, i strap him in and have drink/ snack ready.
We have a 7 seater and he has his seat in the boot where his distraction on my driving is limited, his ability to hit his brother and sister is limited, he can't open a door or window. It is however a safe spot for him.
When we get home DS2 has told me about his day and runs off to play with DD and DS1 is ready to talk to me/ have a conversation rather than just exude built up frustration at me.
The second was recomended by the Ocupational therapist recently as something that works - headphones. With or without music. Just to block some of the stimulation of the world out temporarily. Its like sending him to time out without the naughty step element. 'Why not pop your headphones or for a bit darling and have some quiet time'?
The ed Psyc gave me a really good explanation of the adrenalin/ rage/ rationality curve. This is my loose interpretation...
Its a graph like a hill. As we, humans, start to get an adrenalin rush we're climbing the hill and at the same time we loose the ability to rationalise. The greater the level of adrenalin the lower the grasp on reality. In essence as we loose it, theres no point trying to rationalise with us once we're past a certain point, because the adrenalin has taken over and until it disapates we aren't available for discussion. The adrenalin can be released quickly by our primal reason for its generation - physical exertion i.e. running away from the danger, attacking the percieved danger etc or slowly over time allowing our system to return to normal.
For me excepting that DS1 is not available for discussion during an outburst, whilst looking back is bloomin obvious, means that I can except management is the answer and right now headphones and removal to a safe space work. I'd quite like to try a punch bag out too.
We originally had referals for support via school and our GP.
We have also seen some good community paediatricians who have offered good advice - accessible via the GP.
The school may be prepared to put you intouch with the Ed psyc. I find sending an email outlining any concerns is quite effective because if they think its worthy of querying they can just hit forward.
If you have concerns that there is built up resentment, even if its at a subconcious level, between your DC is there a sibling support group that could be accessible via any support groups that exist for your DS1?
I very much hope for you that its just a phase. I love that bit of parenting advice that gets bantered around 'what ever it is its a phase'.