How can I help my boyfriend come to American for a visit he lives in nigeria

If he wants to come for a visit, then he needs to apply for a visitor B visa at the US consulate in Nigeria. A visitor visa cannot be used to come into the United States with the intention of getting married. If you are intending to get married, you can  apply for a fiancée visa for him, if you have physically met within the last two years. Alternatively, you can get married anywhere in the world and then you can sponsor him for a spousal visa to be processed at the US consulate overseas.