My 13yo ds has many of the same problems your ds has. He is not a fast reader, especially out loud and just seems to not be able to picture/solidify words and their spellings very easily. BUT, he has a great memory for other things, and for that, I am grateful.
I do not critique the spelling in his written narrations anymore, just the content. He is a great oral narrator and becoming a good written narrator (although he doesn’t love writing and especially doesn’t love descriptive writing), but still does what he is told and I praise him for it. He enjoys reading, most of the time, and is moving along in grammar, Latin, and SW. His strong point is not Lang. Arts, but has a great comprehension level, and listening skills.
I used to really beat him up about his spelling, still do but not like I did, when I realized my very fluent, story-telling reader could not remember his math facts for nothing!!! Spelling, grammar, reading (off the hook!), but 5+5, not so much.
I’m not suggesting you give up on your son, on the contrary, don’t give up!! But, maybe work at it in a different way, see progress in him differently than you would your other more natural spellers. Ask him to really focus more (maybe he already is). I had to remind this ds of mine to really focus on the words, don’t skim over it (he just isn’t into LA as much as I am or his brother) as if the words are not important, etc.
Dictionaries didn’t help my son, either. He didn’t know if a word started w/ a “ph” or an “f” so that didn’t help. Extra copywork, focusing on each word, Latin, grammar, and not letting him slide. I have had to “discuss” the reason why a word may be spelled like it is, or how its Latin root may cause its spelling, or without the “E” it’s just spelled wrong or creates a whole other word, etc.
Gotta run. Your ds is not alone, and neither are you!