Since he is illegal in US, he may not be able to adjust his status to get green card if he entered US illegally or his authorized stay is already expired. Assess the consequence before you get married. More details:
http://uscis.gov/graphics/howdoi/LPReligibility.htm#k
and
http://uscis.gov/graphics/formsfee/forms/files/i-485.pdf
http://uscis.gov/graphics/howdoi/LPReligibility.htm#k
and
http://uscis.gov/graphics/formsfee/forms/files/i-485.pdf
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