If my fiance is in the US army and is a citizen can we still get married and fix his mother's papers?

Generally, a U.S. citizen age 21 or older may sponsor a parent for immigration in the U.S. in the Immediate Relative visa category. This is true regardless of whether the U.S. citizen may be in the army and regardless of whether the U.S. citizen may be married. Of course, there are many other details that can determine eligibility, and there really is no substitute for your fiance to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain his goals, and who then could offer legal representation in the often complex application process.