He could very well be dyslexic, but very, very smart and have come up with ways to navigate around it. I work with lots of dyslexic kids. Most can't spell worth a darn. Many can learn to read, and read well, but the spelling is the BIG CLUE that there is something seriously wrong.
My littlest sister (7th grade) is super smart and also dyslexic. She has a photographic memory and has basically learned to read entirely by sight. She can hardly spell CVC nonsense words and makes NO sense of the vowels. It's really been difficult for her. She is smart enough that she is doing 9th grade math, in AP classes in EVERYTHING but needs someone to check her every word or use spell check before turning something in. She has memorized enough words now that she has generally decent spelling, but when she doesn't know a word, first she asks someone (say a science word that's new to her) and then she just remembers it. It takes her awhile to spell it though, but she has enough strategies that she usually looks back in the book.
Something funny: She couldn't spell WHEN for the LONGEST time... like as a fifth grader... AND my step-mom had made WHEN the password to get on the computer. My sister would have to type that to get on the computer every time, and still couldn't remember. When she finally got that word, my SM changed the password to something else.
It's possible that if this boy doesn't have terrible spelling during class assignments he is using the environment (word wall, posters, prompts on the board, etc) incredibly well to compensate.
**** I realized I didn't give you HELP! I would have him assessed if possible and then see if it's really a problem. You could probably do a little test yourself by just having him spell and write nonsense words. See how he does. He probably needs to review the vowels, because they are so tricky, and perhaps needs some intervention. However, if he is above grade level in everything else, he probably won't get the help. Teach him how to use spell check.
And maybe do more writing