Mozeze

The National Population Commission (NPC) has an office in every local govt council headquarters in Nigeria. So, your fiance should go to the council secretariat of the local government area of his residence to find out the exact address of the NPC in his place of residence. They will have him get either of his parents to sign an affidavit of birth. It is quite a simple process, so relax. The birth certificate can be obtained in a day and cost should be something in the region of N3,000.00.
In Lagos, medicals currently take three days, baring any unforeseen health issue. In between the three days for the medicals, he can go to the Force CID at Alagbon and get the police clearance certificate in only a matter of two hours.

Hi Onye Uwaoma, please see an extract from the I-864 Instructions, which I clearly states she has to include both dependants on the I-864 since she included both in her most recent tax returns:
21e - Enter the number of any other dependents. You must include each and every person whom you have claimed as a dependent on your most recent Federal income tax return, even if that person is not related to you. Even if you are not legally obligated to support that person, you must include the person if in fact you did support that person and claimed the person as a dependent. Hope this clears any lingering doubt. Wish you guys God's help.