My Nigerian bf and I want to travel in the us for 3 months and then he’s going back. We just want this short time together because I am not well. How?

Your boyfriend needs to apply for a visitor visa and qualify independently. He applies by filling out a DS-160 at the US Consulate to get his B visa if he doesnt already have one. Your participation in the process is not required. He will increase his chances by also including a letter from his employer, paystubs for the last 3 months and tax returns showing stable income. Also if he owns property, he should submit a copy of that deed along with an appraiser report showing what the equity in the home is.