Autistic Meltdowns at School - how do I avoid my 7 year old being expelled??

My 7 year old son is already on his 2nd school having been managed moved from the first. His first term here went without a hitch as he had an excellent teacher, however since September, when he moved to a new class, things have gone from bad to worse. He has not been to class since October and only goes in for half days, I am convinced he has learned nothing this academic year. We are awaiting diagnosis for Autism but are fairly certain he has high functioning ASD, possibly with a PDA profile.

He has been excluded 9 times since September, all for bad behaviour following angry outbursts, the trouble is that he can't control his behaviour once the red mist descends and school don't seem to be capable of keeping him regulated.

He was given an EHCP in December but so far it has had no effect as the school are trying to recruit a 121 teaching assistant in order to put the measures in place.

When my son is regulated he is academically capable, he is sweet and kind and a really great pupil, but he doesn't seem capable of staying regulated at school and I don't know what to do. He can't be the only child who suffers from meltdowns but school don't seem to have any way of handling it.

Has anyone got any positive experience they could share? Any advice? Anyone in Gloucestershire who can recommend any schools in the area who are equipped to handle this?