We also continuously "sell" the concept of practice. I'm not sure the message sunk in yet with my kids (12 and 8) for things that don't come easily, though we are seeing it seep into the corners of their mentality for easier tasks.
One thing that we as parents try to keep in the forefront of our minds is that there's a big difference between "can't" and "won't," even if they tend to look the same to an observer.
We spent a lot of years being convinced by teachers that my older child was just lazy and sloppy. It affected our mindset about her. After all, she *could* neaten things up when she brought the full force of her brain to the singular task of writing. Indeed, she simply *could not* focus on the writing and the content at the same time. It was not a matter of choice, practice, or determination.
Even with that perspective, we're finding that we're still feeling ourselves slip into this mindset with our son, whose development is looking eerily similar to his sister's.
For the 504/IEP eval- there are many measures out there for prewriting skills. If writing hasn't been taught yet, the prewriting skills still need to be in place to learn the letter formation. Respond with a request for that testing. You have the dyspraxia diagnosis already, so there is reason to take *preventative* action before developing a "I can't" attitude towards learning.