This is a dicey area of law that has changed a lot in the past two years, and will probably continue to change.
Here are some key points as of April 2013 first, and details below:
a. the Post-Conviction relief process, if he is still serving the sentence, or
b. through a carefully drafted 2-1401 motion to vacate, if filed within 2 years of the plea.
In 2010, the U.S. Supreme Court held that it is a violation of the 6th Amendment where an immigrant was not advised by counsel about the immigration consequences before pleading guilty to an offense. That case is: Padilla v. Kentucky, http://www.supremecourt.gov/opinions/09pdf/08-651.pdf Immigrants nationwide have been using this holding as means to vacate guilty pleas. However, the application is severely limited in Illinois, both by statutory limitations on vacating convictions under state law, and by the U.S. Supreme Court decision in Chaidez, that this rule applies only to guilty pleas entered after Padilla was decided.
Chaidez v. United States, Slip Op Feb. 20 2013, (U.S. 2013) http://www.supremecourt.gov/opinions/12pdf/11-820_j426.pdf In People v. Carrera, the Illinois Supreme Court held that an immigrant can apply for post-conviction relief if he pled guilty to a crime due to ineffective assistance of counsel only while he is still serving the sentence. People v. Carrera, 940 NE 2d 1111 (IL Sup. Ct 2010)
http://scholar.google.com/scholar_case?case=8142785412512364471&q=%22239+Ill.2d+241%22&hl=en&as_sdt=2,14 However, attorneys in Cook County have had success using the Code of Civil Procedure 2-1401 motion to vacate instead. This needs to be carefully worded to state that the judgment is being vacated due to a constitutional, statutory, or procedural defect. These motions must be made within 2 years of the judgment, however.