I could not find any evidence that Willis had a son named John, however, his son Samuel T. Coggin had a son named John L. Coggin, born 9/27/1850, died 7/31/1922. He was married to Martha J. Glass and buried in Bethany Baptist Church cemetery in Gulf, NC. Samuel and his wife Salinda are also buried there along with several of John and Martha's children. The grave marker of Samuel lists Willis Jacob Coggin as Samual's father. I am not sure if this is the John Coggin you are searching for, but if so, I found this information at the cemeterycensus website along with pictures of the headstones mentioned with information. Please let me know if I can be of any additional help.