Unlawful Entry Options to Remain in the U.S

I apologize if this is in the wrong forum or has already been addressed. I promised a friend I would try to find an answer for him.

Facts: Mexican national entered the United States illegally in 2003. He was an adult at the time. He had no other family here. Since then he has had a child with a US citizen. He is not married.

What options are available to him to become a legal resident?

Thank you.

Unlawful Entry Options to Remain in the U.S