Len Pincus wrote:

My GGM had a sister. The sister had a daughter. The daughter had a son.
What is my relationship to the son?

My GGM had a son. The son had a daughter. The daughter was my mother.

The grandson of your G-Grandmother's sister is your 2nd cousin once removed.
That man's children would be your third cousin.

Regards,
Michael Fener
New York, USA

Searching:
FENER - Hungary
FULEP/FULOP - Hungary
WEISS/WEISZ - Hungary
MARGULIES/MARGOLIS - Galicia
MICHAELSON/MICHALOFSKY - Russia
KULIK/KULIC/KULICK - Russia