My dd13 has this same issue - she loves to read read read fiction and I am loathe to ever ask her what happened in a book because instead of summarizing she'll recount every single last darned detail - even if it's years after she's read it
She struggles though with remembering factual text that she reads, particularly science. I don't know if this is true for your ds, but my dd remembers things much easier if she either hears them or experiences them visually (not by reading words). Audiobooks might be worth a try, or one thing my dd does is take a small chunk (section) at a time, reads it, thinks it through, then moves on, instead of reading a full science chapter at one time without stopping.
I also wonder, if your ds is reading books that are a bit above his readiness level for non-fiction comprehension, on the topic he's looking at? Fiction and non-fiction are two very different beasts - my dd easily reads fiction light-years ahead of where she's at with non-fiction. So you might try having him learn from a lower reading level book at first, then move up.
One last thought - when my dd started complaining about how she couldn't remember the things she read in science books, I was completely surprised because she reads so far above grade level and is finishing up huge novels within hours of picking them up, with total recall. So I asked her how she was able to remember what she read in the novels she reads. What she had to say about it was interesting and revealing - she thinks in movies, and when she reads fiction, that translates into pictures/movies as she reads. Science textbooks, not so much! When she reads a chunk at a time, then processes it, she's essentially building the "movie" in her head
Best wishes,
polarbear
