Hi claw, just seen this thread, hope I don't just repeat what's already been said.
Sounds like everyone's giving you a really hard time for doing what you know is best for your son. My sympathies! From what you say about how his tutor speaks to him, my son, also, would be getting very anxious and difficult to teach in that atmosphere.
I don't know much about hypermobility as such, but we had similar difficulties getting our dyspraxic son to write, and after practising with him for a year and seeing little or no progress and a very unmotivated, unhappy and under-achieving boy, I decided to give up on handwriting and get him touch-typing. Luckily his brave class teacher supported the idea, so long as we taught him to touch type and supplied the laptop. We've never looked back, it's completely transformed him.
The way it seems to me, asking my son to handwrite is a bit like asking a short-sighted child to read without glasses. Why ask a child with a disability to manage without the assistive technology that would make it so much easier for him? And who handwrites these days anyway?
On the subject of scribing, I think the problem is that it stops children from feeling in control of the task and makes them dependent on an adult, which they'll resent if they're, er, a little bit independent-minded.
I think switching to typing helped my son in two ways. Firstly, he knew very well that his handwriting looked terrible and was illegible. Teachers would have to ask him what he had just written. So that wasn't great either for his self esteem, or for his teachers' perception of him. Like it or not, most people see messy work and judge the child subconsciously by it. Secondly, he was devoting too much of his processing power to moving the pencil correctly, and not enough to the content of his work. So basically when he types, he's free to concentrate on the content, which improves dramatically. Plus he can read his own work back, which helps with coherence and error checking.
On a practical note, we used Nessie Fingers typing program to teach him to type. It's got a lot of quite fun games and helps with spelling, as you can select word lists to practise based on common spelling patterns. We did it a little and often, maybe two sessions a day of ten minutes, with big rewards for progress.
Good luck, whatever you strategies you decide on!