What does he do when encountering an unfamilar word when reading? How's his reading in general?
I would seek out testing for phonological processing and expressive language. We've also had good luck with using graphic organizers for writing to separate the process of coming up with the thoughts and ideas and constructing the sentences. They need to be tuned to the assignment, asking for the right lebel of detail expected, etc.
Google "stealth dyslexia" to see how that fits enough to pursue further testing. My DD was diagnosed at 9 with a profile that's similar, but not exact. I put off testing for a long while because she didn't squarely hit each box on stealth dyslexia. In retrospect, she was compensating a lot.
