You've already gotten a lot of great advice from the other replies, so I'll just make a suggestion again - it's important to get the details of how the assessment is being done at school - ie, is he reading aloud or silently, what type/level of book/story, what type of questions are asked, how are they asked etc. If you could also let us know the name of the specific assessment the school is using that might help us give feedback.
I've had two of my children not test well on reading comprehension in early elementary - I'll explain what happened with them, just for food for thought - it might not be relevant to your ds at all, but maybe illustrates how complicated the answer to your question can be, because there are all sorts of reasons a child might not test as well at comprehension as you'd expect.
I'll also mention that my eg ds who was reading way way waaaay above grade level starting in K was only interested in reading non-fiction until 4/5th grade and even then it was a bit of an odd twist how he was motivated to read his first fiction book that he actually enjoyed. Although he did enjoy Captain Underpants when he was younger
It's been much easier once he was in upper elementary and surrounded by other boys reading fiction to get him to enjoy reading some of the popular series (even though he had the ability to read them much younger). He's also enjoyed the books he's read in middle school too - so I think that part of what many parents of young boys experience in early elementary is simply that there aren't all that many interesting fiction books out there that appeal to young boys as independent readers, but there are tons of non-fiction books for that age range. Non-fiction is the type of thing a child with a high level of reading ability can pick up and enjoy no matter level, whereas fiction books tend to come with more age-appeal for older kids/adults at the higher level of reading that many of our kids are capable of.
Back to our experiences with testing comprehension - my EG ds' school only used DRA when he was in K-2 grade, and they weren't expecting kids to read way ahead of grade level. If your school is using something similar to DRA - where they work the way up through levels and the child has to pass level "whatever" test to get to level "whatever + 1" and be tested there - suggest they start at a really high level just to see if perhaps it's an attention/boredom/lack of interesting challenge issue.
That said, once he got past K-3 level books (DRA type testing)... my ds was not able to summarize what he'd read - not because he didn't understand and comprehend; he's got a memory of detail that's beyond amazing, and if you asked him to retell a story he would, when he was younger, start at page 1 and try to tell you every single darned detail that happened. When he realized that's not exactly what people are asking when you're asked to "summarize" he was stumped, and to this day (7th grade) this is still a skill he has to really work at. There's a disclaimer here - ds has an LD that impacts written expression, and this is a piece of that LD puzzle for our ds, but achild with no LDs might have a difficult time with an instruction to "summarize" if they are a child who remembers things in a very detailed way.
MY HG+ dd who is finishing up 3rd grade has a very subtle reading challenge (she has difficulty with sight-sound symbol recognition). Like your ds, she started reading early and she was at the top of her class in reading for the first year or so of elementary school. Then she started to appear less ahead and more "typical" in her reading skills. Her school uses AR reading tests which ask specific questions and you have to get a certain number correct to pass each test - and dd had the *toughest* time passing the AR tests... until she finally worked her way up to testing at a higher reading level, and now she finds the tests are much easier. It might be easy to chalk that up to a gifted child being bored with the reading and the test, but what was really happening is she's skipping or not understanding individual words and she's reading from context, and it is easier to pull together meaning when you have a lot more story to read.
I also have another daughter who has never had an issue with summarizing or retelling you what she's read. She has a fairly detailed memory and can recall small details in what she's read, but she's also not stumped by the request to retell or summarize. She also *really* likes to write stories. I think sometimes things like this simply are a function of who we are as individuals, so I wouldn't worry too much about your ds not being able to show his comprehension as long as you feel you know he's reading ok. If you have any concerns, or if you think the school is holding him back from where he should be placed because of it, I'd suggest you request (in writing) a full reading evaluation; this would include an oral reading test along with other types of tests - basically the type of eval a student would go through for dyslexia - an eval taht tests a broad spectrum of skills used in reading. If there is something that is holding your ds back, you'll find it this way. ALSO - if your ds doesn't have an issue with comprehension, the benefit to an eval such as this is comprehension will be tested in different ways and you'll most likely see he's a-ok but just has an issue with the one type of test he's taken so far.
Best wishes,
polarbear
