The Average range on the WISC-V (and most other cognitive measures) is 90 - 110, or about 25th %ile to 75th %ile (middle 50%).
Before attempting to remediate his (not really deficient) working memory, I think it would be important to find out why that score is relatively lower--or the intervention may not be effective. Beyond that, there currently is not a pool of strongly-effective interventions for improving working memory per se (although you can spend excessive amounts of money on a brain training program that will improve your ability to score well on specific measures of working memory--but with negligible transfer or generalization to any other specific task).
So again, the first step would be to acquire more data, perhaps through a review of school/academic/behavioral records, or additional assessment in academic achievement, or further neuropsychological evaluation. Just because he scores well on group-administered (multiple choice) standardized tests of achievement does not mean he does not have subtle academic skill deficits--in fact, you've reported one, which is that, despite being an excellent reader, he has struggled with reading comprehension questions for multiple years. This may be boredom, it may be not paying attention to the story, or it may be difficulty with inferential comprehension, which is not actually that unusual in children who also have difficulty with following directions and self-regulation.
