Once you are married, you can petition for approval of an immigrant visa. However, he would not be able to adjust to permanent resident here in the US due to his illegal status and would have to go back to Mexico to receive his immigrant visa. Depending on how long he has been here, he would face a 3 or 10 year bar from reentering the US from the date he leaves. There are waivers for hardship. He can seek approval for a waiver before leaving the US, but would still need to go to Mexico to receive the immigrant visa if the waiver is approved.
Answered on Oct 18th, 2013 at 11:39 AM