Your cousin.
Your cousin in law
Your uncle's son is your first cousin.
It means you have (or had) at least two uncles and at least two great-uncles, and that is all it means.
Your uncle/aunt's son/daughter is an adult and you are still small, your uncle/aunt's daughter had a child, and that child will call you uncle. Uncles/aunts are your dad/mom's siblings.
Your uncle's grandchild would be your second cousin because his grown son or daughter would be your first cousin.
your uncles son is your cousin
His great uncle
great uncle
I call them my uncles
Nieces.
Chachi
Your son is your uncle's grandnephew. Your uncle is your son's great uncle.