I am sort of in your situation at the moment, albeit with a full-time Statement (which actually makes it both easier - as the LEA are making sure the school do all that they can - and harder - as there's not much more that the LEA themselves can put in place).
It's crap, isn't it. J has just moved to the Junior school from the lovely, inclusive Infants, and they clearly want him moved on and have tried to exclude him. I say 'tried' because when they tried to exclude him (for really quite minor, if rude, behaviour) I pointed out that that was their right, but I would formally complain about their discrimination against my son on the grounds of his disability. I cringed even as I said it, but they WERE!
They weren't making reasonable adjustments for his disability, as they weren't following the recommendations from me or from other professionals, and they were excluding him for things that were part of his disability and which he finds very hard to control.
I printed out the following cases from the SENDIST website, which are all examples of cases of children with autism/ ASD/ ADHD, which is what my son has (autism and ADHD), who were excluded or otherwise treated unfairly and whose schools were told off for discrimination.
The HT certainly didn't like it BUT he got the message: that I wouldn't accept an exclusion and that he would have to work harder to put things in place for J.
www.sendist.gov.uk/Public/Upload/DIS0031.pdf
www.sendist.gov.uk/Public/Upload/DIS0032.pdf
www.sendist.gov.uk/Public/Upload/DIS0022.pdf
www.sendist.gov.uk/Public/Upload/DIS0049.pdf
www.sendist.gov.uk/Public/Upload/DIS0046.pdf
www.sendist.gov.uk/Public/Upload/DIS0043.pdf
www.sendist.gov.uk/Public/Upload/DIS0053.pdf
www.sendist.gov.uk/Public/Upload/DIS0030.pdf
He's not yet excluded J but made it very clear that he is furious that I've threatened him. I feel like saying, well, if you're so sure that you aren't discriminating, you'd exclude him despite my threats to complain about Dis. Discrim., so you obviously know that you are discriminating then! But they keep saying that they can't meet his needs and even though the LEA will offer ANYTHING that they need to make the placement work.
The fact that school can't cope too well is good ammunition for a Statement, and you're in a strong position there. It might be worth laying it on the line with this HT (and with the SEN governor if necessary) that you don't intend on removing him from the school, that exclusions would be disability discrimination (unless he does something so horrific/ violent that it warrants the less favourable treatment that makes it discrimination) and so they're best off working with you to get the LEA to provide them with more resources to enable them to meet his needs.Ask the school to call for an emergency review (I'm guessing that he's on School Action Plus) with all professionals involved with your son present.
That's the game I'm playing at the moment - work with me, because J is not going anywhere! Admittedly, I'm in a different position because it's very hard to permanently exclude Statemented children and because the LEA know that this is the only not-atronomically-expensive placement suitable to meet J's needs.
I know what you mean about a fresh start being good. For me, the issue was about J losing his friends (which have taken a very long time to find) and also, why should J have to move when it's the HT's attitude to inclusion that needs to change?
When you say you're waiting for the results of statutory assessment, do you mean to see if you'll get an assessment or to see if you'll get a Statement?
Good luck - I know too well how soul-destroying it is, but the law (and moral right!) is on yours and your son's side. xx