Well, you need to try to gather more evidence like emails, phone recordings, witness statements and whatever else you can get together. The more you have, the better it is. This is important because there are two big penalties for marriage fraud.
The first under 18 U.S.C. § 1325(c), provides a penalty of up to five years imprisonment and up to $250,000 fine for any "individual who knowingly enters into a marriage for the purpose of evading any provision of the immigration laws."
The second is a permanent bar to ever getting Residency again meaning that even if he marries another U.S. Citizen or even if he has U.S. Citizen children, he cannot get Residency and without that, he could be deported.
You can report marriage fraud by contacting the local USCIS field office. Here is a link: http://www.uscis.gov/about-us/find-uscis-office/field-offices
You should have evidence of the marriage fraud handy in case they want to ask you questions or have you send evidence to them.
Don't let him get away with it!
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