I've been looking at the Evan-Moor Daily Reading Comprehension series for one of my own. It's designed for use in 15-minute bursts, basically, and would ideally be briefly teacher-introduced each day, but looks fairly self-explanatory (or he could get the teacher version, which has both directions and full-size worksheets in it, and read the teacher explanations himself, as they're scripted). Each worksheet has a reading passage, followed by one short answer question and a handful of multiple-choice questions. It's laid out a week at a time, emphasizing a particular reading comprehension strategy each week. The reading passages are self-contained for each day. Since it's supposed to be a supplement to a full language arts curriculum, the worksheets are pretty stand-alone, and could easily be done here and there. He could do as many or as few worksheets as he wanted to do on any particular day. There are books from grade 1 to grade 8. It's a teacher resource, which might have more legitimacy in the eyes of the school.
