Hi Michael, Good to hear from you. FAIRBAIRN researchers have much more than this forum. There's even a Surname DNA project that can help overcome the lack of paper trail and prove, or disprove, assorted family stories, albeit not tell us exactly how two matching people may related, just give indications as to how many generations back their most recent common ancestor was.
"last surviving biological grandson" is the sort of statement that says to me, last chance to grab some dna and do your bit for your family genealogy and the overall cause of FAIRBAIRN research.
So far the project has results in, or in process, for 20 FAIRBAIRNs, and by far the majority, (15), are showing up as related somehow, all with paper trails that place their ancestors on the Scottish Borders. The project is starting to show another grouping around those originating in Fife (FAIRBAIRN) and those who can trace their ancestors back to Ireland (FREEBORN). The better, more up to date, site for this sort of detail is the DNA Projects Portal http://dnasurnames.info/lineages/spFAIRBAIRNLineages.htmhttp://dnasurnames.info/lineages/spFAIRBAIRNLineages.htm
I'll send you more details about such testing privately in the hope that you may be interested in doing your bit for FAIRBAIRN researchers everywhere. And I can send you what info I have on your family in the hope it helps.