Have you spoken to your GP at all?
My ds pretty much had a breakdown when he went into Y4 and that led, ultimately, to him being diagnosed with ASD just prior to his 9th birthday.
Everything from lesson structure to the social arena changes when they start the juniors and it's a very common point for children who have been masking or holding things together in infants to suddenly stop coping.
It's good that the HT has asked for a referral, but the quickest way to diagnosis for us, was to circumvent the school and go to the GP, who referred to a Paediatrician, who then referred to the ASD assessment unit for a multi-disciplinary assessment. Diagnostic pathways are different from area to area, but I would definitely speak to your GP about what's possible and where they can help. I was also able to refer ds to the Educational Psychology Crisis Team, but afaik, parents can no longer self-refer to them and I understand that it was pretty rare in the first place, but might be worth checking.
Then, as Yourarent said, you need a meeting with school to discuss what specific strategies they are going to use, whilst referral/assessment is taking place to ensure your ds is properly supported, so he doesn't reach the point where lashes out etc.