I would concur that reading is easier to challenge oneself independently (I'm having trouble making this sentence work, leaving it for now). Although math is more my forte, since I never had acceleration (aside from being in an "advanced" math class from 4th through 7th and taking honors/compressed classes in high school and skipping precalc), it took much longer for me to develop my mathematical knowledge base and skills just having to teach myself. Sure, I learned some calculus when I was somewhat young, but I didn't have a rigorous understanding, and had many gaps (it was easy enough to pick up on polar coordinates and certain functions, but I lacked understanding beyond plug-n-chug type problems in calculus, even though I taught myself formal proof writing from a college number theory book). In English, however, I learned very rapidly without much assistance, primarily due to the existence of libraries and the Internet. I got to discuss things like philosophy (epistemology is a favorite) and any other topic that piqued my interest, like psychology, sociology, etc.) Once you reach a certain ability to read, the options to learn about the world widen dramatically, and since language is a form of communicating information, it is useful for learning about many things independently.
Now, I'm not a teacher, but when I was in school, 5th-8th grade seemed notorious for bullying primarily because of the age group. But this situation may be more so than normal or have more to do with particular students who will graduate in a year or two, so that's tough to weigh in on. Also, since you have personal and professional understanding of gifted students' needs, you may be best suited to provide useful differentiation - if not much in instruction, due to educating the other students, then through having alternate assignments - recommending a list of books that are more at his reading level, but with multiple books to choose from, so his interests can lead him to want to exercise the reading skills.
This sort of set-up worked fantastically in my 7th grade language arts class, where we had a long list of books from reading levels of 5-6 to reading levels for late high school and college. So students got to choose their books from a number of the levels (though to discourage a high ability student from trying for an easy A reading an easy book, the further below your measured reading ability the book you chose, the more work you had to do for the project to get the same corresponding grade). It wasn't terribly formalized, or strictly numerically calculated, but it was more of a guideline to encourage students to challenge themselves by reading something interesting while discouraging slackers (like me).
As long as it isn't a lot of extra work, and it seems interesting, it'll probably be at least all right, if not a tremendously enlightening experience. In teaching, as in medicine, the principle of "First do no harm" is very apt, so even if you struggle to meet his needs, that's the really important thing - if you can help keep his work ethic and interest in learning alive, that's a job well done IMO. For basic tasks that might frustrate him for their basic quality, perhaps you could use a system where after a student has demonstrated proficiency in a task, they don't have to keep doing assignments on it and move on to something else (whether reading, a worksheet on a different topic, some other activity, many things would work).
That way, he could be excused from repetitious exercises but not draw the ire of other students who think you're just making a special case for him to not do the work. A little formalism probably wouldn't hurt - like, after you do X amount of this type of thing correctly, you don't have to do the rest of it, and can instead work on one of these activities. That sort of thing may work better for math, where the specific tasks are segmented a little more clearly, but depending on the level of English and the type of activities, something like that may work.
Sorry if I'm just saying things that are obvious to an experienced teacher or if what I'm suggesting is utterly impractical - but some good ideas have eluded experienced teachers before, and I am known to advocate for impractical things.
