I have no concrete leads to offer you, but since I'm working on a similar problem in Rebecca Coffee (born 1774 in South Carolina) I thought I might at least offer some suggestions that might lead you somewhere.
There are a number of Coffee families in the Lancaster area of South Carolina at this time.The best represented family in the online genealogies seems to be Hugh Coffee (married to Agnes Montgomery).Hugh is the son of John Coffee and Susannah Watson formerly of Virginia.John and Susannah had several other sons who come to South Carolina when the family moves.Those sons, other than Hugh, include Henry (b. abt. 1748), John (b. abt. 1752), and Nathaniel (b. abt. 1754).I keep hoping to run across information about one of these other sons which might help me unlock the key to the origins of Rebecca Coffee (who married Robert Montgomery).Rebecca and Robert moved to northern Mississippi (Lafayette / Calhoun County and Pontotoc County).Perhaps my Rebecca is the older sister of your John.If you find any leads which tie the two together, I'd appreciate the help, and I'll certainly keep you in mind as I hunt.