Can my boyfriend become a legal citizen if I marry him and he has had his green card for more than 8 years living here?

Generally, a foreign national who has lived in the U.S. for more than five years as a Permanent Resident (holder of a "Green Card") may succeed in a petition to become a naturalized U.S. citizen, and this is true regardless of whether he may be married. Of course, there are many additional details that determine naturalization eligibility. There really is no substitute for you and your boyfriend to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain immigration-related goals, and who then could offer legal representation in the often complex application process.

Your boyfriend doesn't need to be petitioned if he is a lawful permanent resident ("green card holder"). He can file for citizenship by naturalization on his own if he wants to, but may get deported if he has a criminal background or has violated a civil immigration law. He must decide what to do at this point and may want legal advice before he files an application. The above is general information, not legal advice, and does not create an attorney client relationship.