Cola, one thing I have personally found really helpful (because of the way I think), is writing it all out. I have done this for both DCs, producing a pre-assessment note for the psych (which both times turned into more of a thesis, LOL), basically outlining "why are we here?". I usually gave myself a couple of weeks, and nibbled away at as thoughts occurred to me. Having DH produce some independent notes to include was very helpful as well. I also was doing constant research on many gifted and LD sites and issues, which triggered constant additions to my notes, usually of observations I wouldn't have thought significant until seeing them in these new contexts.
Both times, by the time I was ready to send the notes to the psych, I had a really different understanding of what, well, I understood, if you know what I mean. Patterns I hadn't recognized, clues leading in a consistent direction when you line them all up - it was a hugely educational experience for me. I had started out bewildered and stressed by my inability to figure out what my child needed. By the time I was ready to provide the notes to the psych, I had opened up my own eyes so much, that what she ultimately told us came as no surprise.
For example, based on my preliminary research, I had concluded that whatever DD was, it clearly *wasn't* dyslexia and it *wasn't* ADHD. After a lot of cycles of digging, writing, thinking and digging some more - especially around this forum - I realized that when mixed with giftedness, these descriptions might, after all make a lot of sense - and she was ultimately diagnosed with both. Labels can seem scary, but I think the diagnoses are both accurate and a huge help. As a wise person recently reminded me, "I need to know what the problem is if I'm going to be able to fix it". Understanding the underlying reasons why your son is having trouble - not just the surface triggers like "show your work" - is essential to making the changes he needs to be happy.
Through the forum, you have already put together a wide range of info and insight about your DS, on many issues. Perhaps it might be helpful to collate it all in one place, try to find some common threads - things that he loves, situations that cause problems, fixes that work, things you've tried, when and how, and what the results were, early observations and what's changed, or not, over time - use whatever clusters make sense, and if need be, keep reorganizing until things kind of go together in a useful way. You've mentioned a lot of school changes; perhaps try to capture the before, during and after of these situations. Everything about these kids affects everything else. It's not sensory issues or the teacher or the curriculum or dysgraphia or ADD or, or, or - it's whatever complex (witches brew!) of ALL these things the kid is experiencing at this time, and over their lives so far. So try to put all those many complicated trees together in one place, to help find the forest and the patterns in that bigger picture.
My way is writing, yours might be lists or diagrams or mind maps or whatever - use an approach works for you, and helps you make a bit more sense of the vast array of ideas and concerns that currently feel so overwhelming. Looking back, I guess this is how I pull together my research work to produce my own thesis in DD-ology! And try and share some of that expertise with the specialists, who can be a huge help - but are only as good as the information you help provide them. Big hugs, Cola, and hang in there - you can do this, but you need help. Reach out, and give them everything you can to help them be able to help you and your son.