Since I last contacted you I have found a lot more information on my branch of the Unions, including details of their emigration from Greenock to the USA in the period 1902-1908. By 1908 some at least of the family lived in Newport News, which adjoins Hampton Va. It's quite likely that your grandfather was the son of one of them; the sons were William, Samuel, Nimmo, Charles and Wilford and the youngest was 24 in 1913 so any of them could have been your great-grandfather.
See also new entry posted above. I'll keep you informed if I come up with any more information.