James Jr. supposedly had a son named David (probably the eldest), who would have been born some time between 1775 and 1785; however, no one appears sure of the relationship. This David lived in upper east TN in the early 1800s, and your David may have been his son. I don't believe any other sons of James Jr. except George would have been old enough to be his father, and George lived mostly around Roane County, TN, and southward. I don't know whether I can be of any real help, but I'll give you what little I know. I'm not descended from James Jr., but I more or less keep up with the web postings for a cousin who is.
I have run across the family of David and Isabel Alexander in Johnson County, IN, when looking there for Thomas D. Alexander, born about 1795. Thomas D., who was a Methodist minister (although most of his family was Presbyterian), was certainly James Jr.'s son, and moved to Johnson County, IN, about 1828, and David moved there about 1833. As I'm sure you know, David and Isabel had a son named Thomas D., and David apparently had a younger brother named Thomas. Although none of this proves anything, it is suggestive.
I'll try to help more if you think that this is leading anywhere worthwhile.
I understand from the cousin I mentioned that one of Thomas's descendants who was probably the authority on his family recently passed away.