We are experiencing the same w/ our First grade girl, who just started in a Progressive school after being in Montessori for five years.  Some kids are very adaptable, but for my daughter it is a huge change that may take the entire year.  We have the exact same issues w/ difficulty in reading comprehension, "details," writing and math differences.  The school felt initially, that we should have a, "language evaluation," which we felt best to hold off on initially, to simply allow her more time to adjust before getting into testing.  Although she seems to be gradually getting the hang of things, she needs much more time and one on one instruction for new concepts.  We are trying to support her at home with organization techniques that she never had in the, "prepared environment," of the Montessori classroom, and in talking about literature, which also was not done at all.  It is my feeling that the right side, (3-d, conceptual, tactile,) of her brain was well developed through her Montessori experience, and the left side of her brain, (verbal, language, organization,) now has to catch up.  Her teacher relied on the Montessori materials very much and was less of a "guide," in challenging her and helping with areas that needed work.  She said that my daugher was very much a "Montessori kid," and worked very well in that environment.  Of course she misses her old school where she could do whatever she chose all day, but she was not challenged where she needed to be and was left without some basic grade level skills.  Two websites that I found helpful with ideas to support areas of difficulty were:  www.visualspatial.org, and www.dianecraft.org  If I could, I would try to draw the parallels to how things like Math were done differently, and maybe it will help his confidence in learning a new system.  Good Luck!!!