At the time your grandfather left, modernday Lebanon was still a part of the Ottoman Empire. More info on the Ottoman Empire and the creation of Greater Lebanon (the predecessor of modern Lebanon) on:http://en.wikipedia.org/wiki/Ottoman_Empirehttp://en.wikipedia.org/wiki/French_Mandate_for_Syria_and_th...http://en.wikipedia.org/wiki/Greater_Lebanon
Also an 1845 map of the Otoman Empire, which I guess must have been pretty much the same in 1890:http://www.euratlas.net/cartogra/ottoman_1845/index.html
By clicking on the map you can get detail parts of this map.
In your case:http://www.euratlas.net/cartogra/ottoman_1845/ottoman_map_7_...
Find Bairout (coastline), right side of Bairout you find a green line with the name Anti-Liban. On the left side of the "i" of Anti you find Zahle.
As for research in Lebanon, if no one comes forward with better ideas, you may want to contact the National Archives of Lebanon, adress on http://www.can.gov.lb/
It may be a long shot, but you never know......