Try and visit him in nigeria first, then come back to US and told for a fiancee visa for him(k1 visa).. but you have to be sure you will be getting married to him within 90 days if he gets here on k1 visa..
Thank you for your time
Please go through USCIS.GOV for this,most likely a B1B2 visa or visitors and tourist visa is what you should apply for since you are yet to get married.The Nigerian consulate cannot help you here☺
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