I would go for a diagnosis. Your kid already knows he isn't as proficient, and knowing why will give him tools to process and problem solve when tasks are difficult for him.
Beyond that, I'd like to suggest looking at accommodations a little differently. Yes, all that writing helps, and he manages. But think about what he could do - and what it would do for him - if he not only managed but excelled. Accommodations do that. When my son dictates a story, it's far beyond what his peers are writing, but when he writes or types it, it barely qualifies as a story. That does wonders for his confidence and for the way his teachers and peers view him.
And just so you know, my son does the same thing with his pants and button shirts. He wears track pants to school and skate shoes that are untied. He wears a hoodies all day that remains zipped, because he is worried about losing it if he takes it off or not being able to zip it again once it is unzipped.
