Hi Ocelot! Great advice from Platypus and Portia here.
I second the idea of bridging him to the more difficult/distasteful activities. Find a way to bring out some of his strengths as you work on deficits through an evidence-based program.
My DS7 only just started writing with any volume or quality this year. Previously, his handwriting was a hot mess, and he would shut down and become angry if asked to write. Through a series of observations where I attempted to isolate the core issue, I was able to determine he was anxious and self-doubting, but that was secondary to lacking confidence and fluency in the mechanics of writing.
Here's a thought. You've expressed concern that, "if he does get a diagnosis of dyslexia, he is not going to cooperate with any evidence-based reading remediation programs." I wouldn't treat that as a foregone conclusion. He might not be ready to jump in with both feet, but you might be surprised at how tolerable you can make the work. With a sensitive instructor (possibly you), he may well make significant progress in an evidence-based program that is adapted to his unique profile and interests.
What if you just assumed he is dyslexic and progressed through an O-G program with him anyway, regardless of whether you test or not, making it fun and as light as possible? It doesn't preclude testing, but it might give your son the confidence and skill he requires to be tested accurately. The worst case scenario if he isn't dyslexic is that he receives a solid grounding in phonics.
With my son and writing, like Platypus, I incorporated a wild mix of instant-gratification with the practice. Our initial sessions were short -- 5 minutes maximum, on a timer -- and we played loud rock music or metal, ate candy, and danced or launched pillows at each other literally every time he wrote a word. I sat with him and, word for word, executed each task with him in real time. Generally, I would demonstrate writing each letter several times and ask him to imitate me. To de-program some of his anxiety and self-recrimination, I'd also make copious errors and ask him to coach me through it sensitively. By being able to guide me, it subtly gave him permission to be more gentle to himself. We're often kinder to others than we are to ourselves.
Fast forward 4 months, and he's now writing fluently in cursive without missing a beat, and sees himself as a strong writer (he is!). What would once have taken a week now takes 8 minutes. Don't be afraid to go slowly to build confidence. These skills are built through marathon efforts, not sprints.
I also can't recommend cursive highly enough for writing if printing is onerous. My DS has always been a fast, whole-word reader, or at least someone who quickly chunks syllables, so I've wondered if he's somewhat dyslexic. He still struggles with printing and reversals in his letters (and some numbers), but has no difficulty with cursive. The letters are more distinct, and the flow of connecting letters to words seems -- to me, at least -- to promote some deliberateness and planning in writing.
