I have a boyfriend who resides in Nigeria and I live in The US and I want him to visit me soon so we can decide on our future.

Your boyfriend should apply for a visitor visa at the US consulate in Nigeria. After he enters the country, if he changes his mind and decides to marry you, then he can adjust status to get his green card without having to go back to his country.   

He would have to apply for a visiting visa and it would be in the discretion of the American consular officer whether to issue it or not. Your boyfriend would have to demonstrate nonimmigrant intent in the sense that he would have to convince the consular officer that he does not intend to stay in the US, but will return to Nigeria once his nonimmigrant purpose is finished. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.