I have no idea if this should go here or the literature section but I've been thinking hard all day what the answer is:

What is the proper saying to refer my grand uncle's son? A second cousin is my parent's cousin's child while a child of my cousin is a first cousin once removed. However I've searched yet can't find the proper term for my grand uncle's son(the son of my mother's uncle).

Found it after an intensive google search. Guess I wasn't specific enough the first time.

"Grandparent <=====> Great Uncle [Sometimes called grand uncle] are siblings
Parent <=====> Great Uncle’s Daughter are 1st cousins
You <=====> 1st cousins once removed"