It might be that he has a harder time spelling while writing vs. spelling verbally. I was not (am not) a natural speller at all, but I could spell out loud easier than spell when writing. I think it was because when I would write I would write faster than I was thinking and honestly just did not see the errors. What my mom often did was to ask me to spell the misspelled words outloud and then just let me go with that (I was homeschooled growing up). She knew that spelling was a struggle for me, (she wondered if I had a form of dyslexia even though reading was not a challenge, since spelling was) and drilling and drilling was not going to be the answer.
What also helped was that my mom would ask me to read aloud what I had just written, that often helped me hear the errors because reading back what I wrote caused me to slow down and realize that something was wrong with the word even if I did not quite know how to correct it.
As I got older with practice and maturity I started to recognize when something was wrong, but did not always know how to correct it. As an adult I am still not a strong speller though I can see the errors much easier now, and I tell the kids often “I don’t know how to spell that let’s look it up in the dictionary.”
I think there is something to be said for maturity and time when it comes to spelling. My oldest did not start really improving spelling until she was 11.5, and my ODS who is 10 and does dictation, will spell words wrong during dictation but when I ask him to spell it out loud or on a white board with out pressure of dictation exercise, he will get it right. Taking what is heard and then writing it down is a very different exercise and takes practice.