OP, the school is severely letting you down here. They should not be laying this at your feet, bundling DS out the door saying 'this isn't acceptable'.
You need to meet with the SENCO (Special Educational Needs Coordinator) and / or Behavioural Coordinator to discuss this.
What support are they going to put in place?
What strategies are they going to use?
What lessons have been learned for future reference (certain triggers, signs of distress etc)
Has DS had the opportunity to apologise to those he has hurt? Has the air been cleared, so to speak, and further ramifications been monitored (possible bullying, withdrawal from social situations etc)
What is the current reward and discipline procedure in his class? Does he respond to it?
Was the teacher made aware of his needs (passed on from previous teacher?)
What level of extra support does the Year One class currently have (eg a Teaching Assistant for half a day, 5 days a week?)
What is the general feedback on your son's progress- is this behaviour an obstacle to learning?
Are there other incidents you have not been made aware of?
Ask to see their Behaviour Policy and Inclusion Policy - get copies.
Ask what they do or have done for children with similar needs.
Ask if they think other professionals should be called upon yet (EdPsych or speech therapist again)
This is most definitely not for you to sort out by yourself - it should be a supportive, collaborative home-school relationship.
Fwiw, Ofsted would an very interested in the way they have handled things thus far.
Take notes, keep records, have someone with you in meetings if possible (and btw- not DS for the more adult-based ones) and hopefully take things from there.