Without being married, the only thing you can do is get him an experienced immigration attorney. There are not enough facts in your posting to determine what he can immigrate based on the petition for her mother. If you were married, you could petition him, and, depending on whether he has a criminal history and what that is, that could be a defense to deportation.
Bottom line, you need to retain an experienced immigration lawyer.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
You need to meet with an immigration lawyer to see what can be done to help the father of your child. See the link to the American Immigration Lawyers Association below. He may be a good candidate for the exercise of prosecutorial discretion to prevent his deportation. If he was inspected and admitted by US Customs when his parents brought him here, a marriage to you might help him stay. If he crossed the border without inspection and admission, but is not yet 18 1/2 years old, he may be able to take voluntary departure and obtain an immigrant visa in his home country through marriage to you. If he's 21 or over, his mother's I-130 approval will not help him. There are many more issues about this case that need to be discussed with an immigration lawyer, and any reasonable consultation fee will be money well spent.
I agree with the other attorneys. Things don't look good with the facts you put in your question. Go visit an immigration attorney.
Franco Capriotti - Senior Legal Counsel - Capriotti International Law
SKYPE CONSULTATIONS AVAILABLE - firstname.lastname@example.org - www.capriotti.com
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If you are low income and feel that you can not afford an attorney, try contacting one of the organizations on this list: http://www.justice.gov/eoir/probono/states.htm
Contact the American Immigration Lawyers Association www.aila.org for a referral to an experienced immigration attorney. NOTE: They also have referrals to low/no-cost attorneys. LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.