I remember my teacher would give me special assignments. Everyone in the class would have to turn in small book reports to showcase our reading and writing skills, but my teacher would assign me more challenging (and interesting!) books, both fiction and nonfiction. He would ask me to answer more challenging and complex questions in my reports. This probably depends on the personality of his teacher, but s/he might have taught children like your son before, and have some ideas for keeping him engaged.

For me, reading was the best coping strategy, with sketching as a close second.

I would also note that most LessWrong readers probably could not have used the internet when they were 8 (unless the people here are younger than I thought) but there are tons of (free) educational programs your son might be able to be a part of online. Online classes, online language programs, Codecademy, etc.