No.
Your internet boyfriend might be in Nigeria, but it would be because he is a Nigerian con-artist, not a member of the US military.

100% SCAM
The ONLY U.S. Military in Nigeria are the Marines guarding the Embassy and they are NOT contacting strangers on Facebook or dating sites
It's a Nigerian scam
Google Nigerian military scam, it's very common
Cut off contact ASAP

Besides nobody would ever be deployed from Afganistan to Nigeria. After a tour overseas you are always returned home. You are never sent directly to another country. Anyone who served in Afghanistan would be sent back to his base in the U.S.
There are no US deployments to Nigeria. The Nigerian government only allows soldiers from bordering countries to cross into Nigeria while guarding border areas. They don't allow foreign troops to be deployed in Nigerian soil

The US doesn't have troops in Nigeria. You're getting scammed,

Are you in contact with an internet scammer?