First let me start with regard to scammers... He is not one. And we've been together on more than one occasion over 3 years .

My my fiancé is currently in South Africa on an expired visa from Nigeria now. My job has returned me home to US. I know his family, friends, and he knows mine, belong to same church etc. Before finding out my job and time there was ending, he proposed and I said yes.
He is going to apply for asylum in SA. How will (will it) affect the K1 Fiancé Visa for US. We have been together for quite awhile now, I am in fear of his return to Nigeria as is he and we are having a child soon. Do I return and marry in SA and spousal visa, or or apply for K1 and hope he is home in-time for birth of our child .

I'm not an immigration expert, but my first question would be on what grounds does he think he would qualify for asylum?

It takes months to process either K-1 or K-3 visa. For some countries it could take up to a year or longer. He needs to make decision where he wants to get settled while waiting for visa. By the way, he needs to maintain his passport, it has to be current.