Second preference (F2B) - unmarried sons and daughters (21 years of age and older) of lawful permanent residents.
23,400 green cards are available each year
the divorce has to be real and not done just so his father is able to petition for him. (See Matter of Aldecoaotalora, 18 I&N Dec. 430 (BIA 1983)).
Some individuals have attempted to hide their marriage status when applying for immigration. For instance, if an adult son of a legal resident gets married after attending their visa interview but before entering the U.S. with an immigrant visa, it may go unnoticed. However, the truth is likely to be revealed eventually.
The most obvious way the scenario above gets flagged by immigration is when the adult son arrives in the US and then files an I-130 petition for his wife so she can come live with him, or he ends up getting divorced and remarries years later and he petitions for that wife (which requires providing a copy of the divorce decree from his first marriage). These types of things also tend to come up when applying for citizenship, since it provides immigration with another opportunity to catch any errors they made or fraud people committed. It’s not uncommon for people who had been living in the US as legal residents for over a decade to end up in removal proceedings after they apply for citizenship and their fraud is discovered.
Engaging in fraudulent activities, such as concealing a marriage, can be a reason for being removed and barred from ever obtaining an immigration benefit in the US in the future. It is also illegal and generally frowned upon.
In sum, don’t lie to immigration. It’s a bad idea and you will probably get caught.