While Jsmitten is likley right--the laws inTN are probably in a state of flux on such matters. The old view was to protect the child against being labeled and to essentially cast in concrete a child born of the marriage as a product of the husband even if science proved otherwise. Some states allow for late debates among unmarried parents, less so among married parents.
The mother may or may not have a right to seek to name a new dad at this late date. I believe Jsmitten practices in FL --he is generally right on target, but even so, brother would be wise to triple check with skilled counsel in the relevant state where mom seks to addres alternate paternity.
As an aside--I suspect it is ultra hard for the husband even if he is now the ex or estranged husband NOT to provide full CS to any child born of the marriage period as I suspect it it now far too late for him to dispute same!