EmmaL, it sounds like you did amazing work throughout a deeply challenging meeting. Well done keeping calm and keeping them on point despite their strenuous efforts to be anything but.
It feels presumptuous for me to try to offer advice in a situation more complex than I can even begin to imagine, but in the spirit of making sure you know we're listening and want to help, here's a few thoughts.
With your circumstances, your question "How do I help DS regain confidence/self-worth?" is tough. The simple answer is to find places where he can succeed, and see for himself that he is succeeding. But from the way he's been desperately trying to avoid school, it's clearly a negative environment for him, and contributing to his low self-image. The long school day makes it hard to add anything else in.
So I can't really extrapolate from our situation to yours, but for what's it's worth, here's some observations from our own experience. For my DD, taking reading back to the basics with a structured program was a HUGE confidence builder. From academically broken in grade 2, she discovered she could learn, and she could learn well, when taught the way a dyslexic needs to be taught. Our home program provided reading materials carefully constructed to only include words she was capable of reading at that point, and she started to discover she was capable. We stopped asking her to do things she'd never been properly taught and couldn't do. After years of nightly battles over school reading, I was the one crying the day she came to me and asked if she could read me a story in her reader, please? Her teachers couldn't recognize this newly confident, engaged, non-surly child.
My (extremely visual spatial but writing disabled) son was born doing math, but learned at school that he was no good at it. His anxiety hit the roof in grade 4, when he found himself unable to keep up with classmates and produce the volume and kind of output school demanded, in math and everything else. Advanced math activities outside of school have been critical for teaching him how to learn, persevere, and see what he is capable of. At home, I scribe all our math on a white board, so his writing doesn't stop him from being able to challenge himself with complex concepts. His school marks are consistently mediocre, but in extra-curriculars - programming, math, piano - he escapes writing and the linear world beloved by teachers, and he excels. After a couple years of him telling me he hates math and is no good at it, this summer he asked to go to an advanced math camp, and found it absolute bliss.
I can only imagine your son has spent years internalizing a horrible sense of himself, and it will probably take a lot more than easy fixes like these to begin to undo. I don't know how, in your circumstances, you can create space for your son to learn at his level - both at the basics for his weaknesses and conceptually advanced for his strengths. Is there any possible way you can come into his school and take over a period a day to tutor him yourself? Could you pull him out of the double curriculum for now, to create time to work in areas that need remediation? Perhaps others can offer some additional ideas for how you can create for him the space he needs to learn what and how he needs to learn.
Sending all the support I can; keep up the amazing work you're doing for your son.
P.S. If you don't need the phone capability, an iPod is a cheaper alternative. Some additional great uses are taking pictures of the blackboard etc instead of writing notes, or recording instructions or information from the teacher. My DD (also maybe dysgraphic?) finds writing on whiteboard much easier than paper, so she often does short work on a whiteboard (instead of iPad or computer with word prediction) and her teacher takes a picture to save her work.
