If you are a U.S. citizen, your fiancé should be able to apply for a K1 visa. Then you'll need to get married within 90 days of entering the U.S. I'm sure you were planning on having some type of celebration of your marriage, but I highly recommend getting legally married asap. Once married, your fiancé will be eligible for a green card. Married green card holders can get citizenship after 3 years, so it's definitely helpful to get the clock ticking.