I have a neighbor who's 1st grader (who just read all the Harry Potter books) has a similar issue. I don't have any particular suggestions. I had somewhat of the same problem with my son last year. His first grade teacher kept him at the same reading level the entire year despite him working through many reading levels at home. I think she was asking for writing skills from him that just weren't there yet in terms of describing plots (which she never explained to him either).

Anyway - I found this interesting article for my neighbor I thought might be relevant. Basically what I think it says is boys can fully comprehend something without necessarily being able to regurgitate easily what they just read. I know my son isn't the kind of kid to pick up something and spend hours on something he doesn't get or comprehend.

http://eideneurolearningblog.blogspot.com/2009/03/why-boys-need-alternatives-with-reading.html

It might be interesting to compare his "comprehension" in a chapter read to him vs. a chapter he reads on his own as well. Our neighbor has tried more oral reading to help with her first grader.