'its how the school deals with it that makes all of the difference'
Exactly this. This new school won't solve all your ds' problems but it will hopefully become a safe and stable place for him which will give him a baseline to work from. He has got a long journey ahead of him regardless of the path he takes, so you need to choose the smoothest from all of the options available.
DS' behaviour got worse after about a term in his new indi school. I suspect like your ds, he is able to 'perform' for a limited period but when the situation became his ongoing reality his guard dropped and I was beside myself worrying if it was all going to fall apart.
I was hauled into the HT's office, the behaviour person, head of house, SALT and his class teacher were there. They painted a pretty horrible picture and both me and dh were expecting the final sentence to include 'please take him somewhere else', so I got in quickly asking if he needs to leave.
They looked at me shocked and said 'no, we are just trying to reassure you that he is absolutely in the right place, we recognise the behaviours for what they are, we are trying to get to the bottom of them but felt we ought to keep you informed of what was going on'.
Despite this, the school is far from perfect. No school will be. There will most likely still be some grating as you and the school have differences of opinion about what he is capable of, how things should be handled and as always the issue of home school communication.
But ask. This is your time to ask. Ask the school what happens in a term if he has perfomed well during the assessment and then deteriorates (as he realises it is his long term school and you won't be pulling him out). Make a list of questions like that, really honest and practical ones and start asking.