My son was like that in spelling at that age. He started reading on his own so quickly, I unfortunately failed to give him a thorough phonics education. It caught up with him once his had multisyllable words. Some words just require memorization, but the majority of English follows rules in order to sound them out and spell unknown ones. Some people are natural spellers, but others aren’t no matter how voraciously they read. It’s a myth that more reading and writing alone will improve spelling: not for everyone. Over and over, I’ve known many parents told that-myself included-but who had kids who read heavily and wrote, but it made zero to little difference. So, we go back to basic phonics instruction. Learning the rules, slowly, and applying them.
So, I started him on the Megawords series. It’s designed for older youth with spelling issues. Just follow the instructions of the program ( so get TG). It’s simple and straight forward.
I agree with you about writing less being counterproductive; I made the same mistake. Copywork daily is great. Writing on his own, not so much, IMO. Reason is that he has nothing to confirm that he’s spelling correctly, so it’s possibly reinforcing poor spelling.
Alternatively, in addition to the copywork you provide, let him choose from books or magazines which he likes in order to copy from, too.
My dd used to skip around and go too quickly while reading aloud. So, I gave her the Pathway Readers, Elson Readers, and Reading-Literature readers (by Treadwell from Yesterday’s Classics, also free, I think) to read aloud a couple of times daily. The stories were rich, but the language simple enough so she increased her confidence. I would read alongside her. When she’d start skipping or mispronouncing, I would just have her go back and have another go. It got better. She’s an excellent reader, now. So, you could try the same with him and see if there’s improvement.
As said previously, testing is an option to glean more insight, if you think it would be helpful. I know I should have gotten my dd tested sooner to confirm her math disability. So, it’s worthwhile.
HTH