Just a couple things: there's a difference between cramming and studying. Cramming, as far as I can tell (as a fifth-year student in college, I hope I know the difference) involves learning the material and seeing it for, more or less, the first time not too long before a test. Studying is focused preparation for a test: practicing, verifying mastery of topics and skills, etc. I don't think this is a problem for your son.
Also, in middle school, it's not exactly obvious when tests are going to be. When I was in middle school nine years ago (Catholic grade school), the teachers would teach and teach, and we'd have no idea how long it was going to be before they were done. Then, around two days before a test, they'd say that they think we'd be done by x day, so that will be the day we have the test. We didn't have a schedule or a syllabus.
Does your son study too much? Does he do well on on everything and still spends a lot of time preparing? If he learns the material early by nightly studying, he probably doesn't need to spend too much time reviewing before the test.
Is he actually productive when doing homework, reading, or studying? I struggle with untreated ADD, and I know that I spend a lot longer time periods studying because I have a hard time focusing while studying. If this is the case for him, that might make it seem like he has more to do than he really does.
If it really is a case of having too much to do and too little time in which to do it, I would suggest calling a teacher or writing a letter to him/her expressing concern. Ask that he or she give assignments sooner and ensure that enough time is given to prepare sufficiently.
At the same time, I had demanding teachers in Catholic middle school and high school (college preparatory), and it's just kind of the way it goes sometimes. (I remember passing a note down my row in 5th grade advising my classmates to ask a lot of questions, as I noticed that, on days when this particular teacher got a lot of questions from us, she usually didn't have time to assign the many questions she was infamous for assigning for homework.) High school is bound to be more demanding.
I hope your son continues to thrive under pressure. :) He sounds like a dedicated student.