Changing from Mainstream to special needs school advice.

Hi. I'm looking for some advice. I'm a mum to 3 Children on the Spectrum. All full time statements. All attending mainstream school. My oldest Child is soon to be 9, he has seriously struggled with the work  for about six months. There have been strategies it into place, but nothing has worked. He is so unhappy, he is getting frustrated and anxious at school as he is finding the workload too hard. Mainstream school is just too much for him now. He is getting angry all the time, at school, refusing to participate, throwing things in the classroom. He is the most sweet, normally happy boy. It's just too upsetting seeing him struggle like this. I'm worried that it's making him regress!  Im adamant that this isn't the setting for him now, and was looking for advice, and information on the process of moving schools. Thank you.