Awh that sounds so tough OP, I can completely relate. I am in the same boat, except I have 3 DC so it's quite tricky.
Our DS is certainly at least ADHD and possibly on the autism spectrum.
Sorry I have only read your OP, so you may have answered this, but has your school completed a FACT assessment for your DC?
My DC's school have been really good. I raised a question over the summer (as it had been on my mind that he would struggle more and more the further up the year groups he got), about whether they had noticed any issues at school, as at home it was becoming clearer and clearer there was something going on and he has become increasingly difficult to manage. Within a week of year 2, they have sent me a FACT assessment and advised I can take it to the GP, which I did, and they have made a referral to CAMHS.
I was initially hopeful that a diagnosis would help, but I am worried about how over run the service is, and had read some posts on here about them essentially diagnosing and discharging.
It is really difficult with children like this, we had a meltdown this morning and they are so hard to deal with. My son is currently nervous of school seeing his meltdowns, despite my reassurance that they are just there to help, like we are, so we haven't had too much bleed through there yet, but just after school is a nightmare. We resort to taking him out most evenings as to make matters worse, we live in a flat so I can't just send him out into the garden, and he disrupts his other siblings.
It is so hard, I've been in tears many times over how difficult it is, but you are doing so well and I really hope the school can help you pursue diagnosis.
One thing I will say is that I am surprised no safeguarding info has filtered down to you, as you put it. I thought schools needed to tell you about stuff like this.
Also, are the school putting any measures in place yet? They don't need to wait until official diagnosis, and my son has been a lot happier at school since his problems were noted by his teacher. He gets to have a fidget toy and special jobs to help him keep focused and on track. It's also helped us emotionally, as there have certainly been times to be honest, where I've just thought I am a shit parent.
Much solidarity to you 