If your fiance is currently eligible for the new Post 9/11 GI Bill, then getting married will not make him ineligible. In fact, whether he is married, and to whom he is married, military or civilian, is completely irrelevant to the new GI Bill.
Likewise, if he is eligible for the housing stipend that can be included with the new GI Bill, then the fact that he is married to an active duty service-member will not make him ineligible to receive the allowance. In order to receive the housing stipend, he must be eligible for the new GI Bill, and be enrolled in classroom courses. If he is taking only on-line or other distance-learning courses, then he is not eligible for the housing stipend.