My fiancé was brought to the US when he was 2 and has graduated high school and worked his entire adult life (he's 23) however under a fake SSN. He would like to become legal, go to college (trade school), and do something with himself and for us. He knows no other life than living here and you would never know that he was not legal. I am a US citizen, born and raised. Is it at all possible to have him here legally without him having to go back to Mexico?

Need more details on his entry. Was he brought in using his own passport (lawful entry)? If not, did anyone file anything for him or his parents before April 30, 2001?

His bet bet at this point is to hire an immigration attorney to review his exact status ( if any) and advise on the choices he has

To my knowledge, no. His mom and dad came here illegally. His dad ended up getting sponsored through an employer and was a legal resident but went back to Mexico 5 years later and lost that status and left his wife and son here without doing anything for their status.

He has no status. He has been here for 21 years and has had no hope of having the life he actually would like to work for.

OK, this is my take from your information.

He entered the US unlawfully as a child, as he was too young to affirmatively make any misrepresentation, his entry will be deemed "EWI", for entry without inspection. As an EWI, he will need to qualify for section 245(I) to forgive the EWI. 245(I) ended April 30, 2001.

But, since you say his father eventually got PR, and if the case was initially filed prior to April 30, 2001 and your fiancé was under 21, then under a USCIS memorandum detailing the "alien based reading of section 245(i)", he is grandfathered as a beneficiary.

If you can prove up his entry, his physical presence in the US on December 21, 2000, and if he is grandfathered for 245(I) under his fathers labor certification, and if you get married, he will need to pay the $1000 penalty to allow him to adjust status in the US. You will be scheduled for an interview at the USCIS Field office and you will need to prove up the bona fides of your marriage and your husbands eligibility to adjust status in the US.

A bit complicated, so I recommend that you confirm all the above with an experienced immigration attorney. Good luck!