If someone lived in the US, served time in jail, and then was deported, is there a way to bring them back?

My boyfriend was brought over to the U.S. as a baby, I'm not sure why he never became a citizen. However, he served time in prison and upon release was deported to MX, a country where he knows no one. He signed paperwork w/o having someone specify details of what he was signing. He had no lawyer and no resources, all he thought he was signing was a deal saying he'd serve 50% of his time and then get released. However, he was never told he would never be able to come back to the U.S. He didn't serve 50% of his time, he served almost 9 years out of a 9yr 3mo sentence. What can we do to reopen this case being that it was mishandled and he was steered into signing away ever being near his 3 kids, mother, and family. Can the 'never able to come back' part be reduced to 5 or 10 years from NEVER?