My 10yo daughter has dyslexia and her spelling is very poor. That is common with dyslexics. I am currently using Soaring with Spelling. It gives 6 words a week to work on, all using the same spelling concept. There are activity pages to do through the week before the final test. I like it for a formal spelling curriculum. But, I have to agree with DawnD. Reading and writing is the best way for them to learn. My daughter can spell all the words correctly on the test, but may forget some of them by the next day. It just doesn’t stick that way. We also use Plaid Phonics to reinforce the concepts. That also covers some spelling.
Unfortunately, some children who have this problem may still be weak in spelling as an adult. I was just talking to a homeschool mom after church yesterday. Her son, her husband, and her mother-in-law all have dyslexia. She said her husband still has problems with spelling and so does her mother-in-law, although she is an avid reader. So, i would say just keep your son reading and writing. If you want something that teaches the concepts, by all means use it, but just give it time. His spelling should improve over time, but I wouldn’t get worked up over it.