I think your observations of his language comprehension are still consistent with delays in abstract language, as two years above his age level is still a fairly concrete level (early middle school). I would find the inconsistency between testing and life more significant if he were comprehending on a high school literature level. Reading for pleasure at a high school level does not necessarily mean that he is grasping the inferential and social-emotional aspects of the books. (I read Anna Karenina when I was twelve, and enjoyed it, but had no idea why Anna would want to leave her husband--thus missing the central conflict of the story.)
School-related language tasks do become increasingly inferential, and reliant on perspective-taking and social comprehension, as you go up through the grades. A lot of the writing in the primary years is personal narrative, which can be challenging for students with relative weaknesses in social comprehension, as one needs a certain amount of perspective-taking skill to be able to communicate internal experiences to an outside reader. Factual writing is probably much easier for him--is that often the focus of his areas of interest? I'd watch for the variance in quality and ease of writing products to increase as the years go by, as literary analysis really goes after the areas of weakness.
