Your SENCO sounds like the ghastly Senco in this thread Punx who said on page 1 " that his diagnosis means nothing moat [most] children with asd don't need support in school."
www.mumsnet.com/Talk/_chat/3794871-So-how-do-i-tell-my-ds-to-go-to-school?pg=1
They are not even willing to consider what his individual needs may be. I'm not sure why nobody seems to understand
But they DO understand Punx, they understand 100%, they just can not be bothered to do anything because they have one method of teaching and if someone doesn't fit into that mould, they want them out, so they make life unbearable for them until they leave. They don't care. You have even quoted his senco saying that.
we would anticipate the needs of a pupil with autism would be accommodated by the pedagogical practices already in place'
It's an absolutely arrogant and despicable thing to say, but it's that school's ethos. You can't make them treat him as they should, you can't make them help him, they don't want to help him, Senco's email says that as plain as day.
Translation - we don't care if he's autistic or not, we are not going to implement any adjustments for him.
It's like going to a restaurant with a one meal option. You either eat it or go somewhere else, but even if you have a serious allergy they will not alter their menu. If you can't eat it, you have to go elsewhere.
I'm going to give myself a while longer to see if things improve
Please, put your own oxygen mask on NOW, so you can help him, don't be so run down with it all that you can't help him if something happens. You need the inner strength to draw on, start now to build up your reserves. 
Their attitude isn't going to change, no matter how wrong they are and how right you are. The frustration as a parent is immense, but you need to see the reality of their actions, not what their actions should be. That school (and sadly they are far from being the only one) are NEVER going to do anything to help him, he doesn't fit their blueprint, they ignore all the bullying, they can't legally kick him out because he's not done anything to warrant that but they can and are making things as miserable as possible for him and you (the bill thing plus dealing with his MH) so that he leaves because he can't cope any more.
He says he hates school but does not seem to understand quite how much their lack of support and compassion have caused so many issues
Part of that non-understanding may be because he finds it difficult to put himself in other peoples' positions and to be able to see a situation from different angles. To me this sounds like an illustration of his autism in action. Rigid thinking, inability to comprehend others' stance or points of view, little to no flexibility in opinion, no comprehension of what school should be doing to help him, no understanding that they should support him, they never have, so in his experience he won't expect them to.
What's blindingly obvious to you and I about the situation in school will probably never have occurred to him. If you need him to understand something, you need to explain it to him first, never ever assume he's caught all the inferences of any situation, always make sure he has a rounded understanding.