Hmm. Sounds like he is a sight word reader, which means he has not really developed phonetic decoding skills. That sometimes happens just because those with a good memory and good vocabulary find it to be the path of least resistance, but it may also be due to neurologically-based deficits in phonemic awareness and other precursor skills to word-level decoding--IOW, something of the dyslexic ilk. Either way, it probably contributes to his spelling weaknesses, and may become an obstacle, should he pursue a field in which there are many low-frequency/specialized terms that differ by only a few letters. E.g., molecular biology, pharmacology.
And yes, there is substantial overlap between dyslexia and dysgraphia. Might be worth while to investigate this further. You can experiment with some phonological processing tasks (in more-or-less increasing order of complexity): blending phonemes (c-a-t-->cat), substitutions (cat: change the c sound to a m sound), omitting syllables (cowboy: now say it without the cow), elision (nest: say without the s sound), reversals (say cat backwards-->tack). At his age, he should be able to all of those fairly easily, with progressively longer words, except possibly for lengthy phoneme reversals (the last task). He should still be able to do some, though.
