What are the steps we can take for him to become legal if my fiancé and I are wanting to get married?

After you are married, you will need to file Form I-130 and eventually Form I-485 with the supporting documents. Depending on how your fiance entered the U.S., he may have to go through consular processing.

First, congratulations. Next, the steps depend upon how your fiance entered the United States. If he entered with inspection, then the process is for you to get married, then file the spousal petition and adjustment of status application at the same time. The process changes depending on the facts of your case. It's a good idea to meet with an attorney so that you can fully understand the situation.