My understanding is that he can. Students who are red-shirted and postpone a year of eligibility are allowed to play an extra year post-graduation as long as they're enrolled in a program, e.g. I know of a student who will be competing as a first year MBA student because he was red-shirted as an undergrad.
NCAA athletes have a five year running clock that begins when they enroll; they have four years of eiligibility (4 seasons of play) available to them during the five year running clock. It doesn't matter what their classification is (mostly-there are rules re: grad school @NCAA.org)- many athletes are in graduate school their last year of eligibility. There are different rules depending on sport and division. The NCAA website lists all of the rules. They also have a number you can call to ask questions.
actually there is also a medical redshirt which in some cases have granted a sixth year of eligbility. You should go to the NCAA clearinghouse website and read the rules. It is probably the best place to get information on college atheletics..