I was in this position last year.
I fought for three months, I got as much evidence as I could from professionals, including school, our GP, CAMHS (though no diagnosis as yet), etc., but ACE and SENDIASS were the best organisations who helped me.
I wrote a 75 page report and included everything I'd managed to collect as evidence.
My case was weak, as even though my child is being treat as SEN, he isn't diagnosed yet.
He was also being bullied, and even though that's a weak argument, I said I needed to get him away from the bullies for a fresh start.
The school I appealed to had a much better set up for SEN children, so I concentrated on that too.
I won, he got in.
It was blood, sweat, and tears for three months but I'd do it all again in a heartbeat, because I got him in to the school and he's thriving.
You'll have people tell you you don't stand a chance.
I was told the same.
I ignored them, I worked my ass off to get as much evidence as I could and it worked.
Contact ACE and SENDIASS, they're fantastic and can guide you.
No one can fight for your child like you can.
Start now, and fight him.
It can and does work - I'm proof of that.
Good luck.