I am sorry that you son and you are going through this. I strongly urge you to meet with an immigration attorney to review the options and the ramificaitons of this upon the process. Take care and I wish you all the best, I will repost this in Immigration for your convenience. take care.
Legal disclaimer: The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of Massachusetts. Responses are based solely on Massachusetts law unless stated otherwise.
This requires more than a bit of guidance. This requires an analysis of your son's family and immigration history along with the statute, CIS guidance and/or BIA interpretation. Hire a lawyer.
If no immigration papers were filed then USCIS would be hard pressed to make any finding of fraud in a subsequent immigration filing. The standard for a fraud finding is that the alien got married for the sole purpose of gaining an immigration benefit. Now (and this is important) just because USCIS would be legally wrong to say this later, it doesn't mean they won't, and your son may find himself in a fight. It would be a fight with amateurs, and one that he would eventually win, but a fight nonetheless. I don't think there's a problem with getting the annulment, but be prepared for some turbulence down the road. Get him a good lawyer that has a reputation for litigating cases in court.