His Grandfather was born in New York City in 1893. Can anyone give me a place to start my search?
Have you got a copy of Grandfather's birth certificate?
If not, and you can't find it, then if I were you I would start by looking for Grandfather in the 1900 United States census. THis should tell you the first name of Great-grandfather, and and then you can look for a marriage certificate that might tell you the names of Great-grandfather's parents.
Or see if you can find Great-grandfather's immigration or naturalisation records.
Once you have that information you are then ready to start looking in Shetland. But it's not a common Shetland surname, so you may find that you have to move on somewhere else.
You can access the 1900 USA Census at
www.familysearch.org