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
That would be your cousin, honey. Just keep it simple and call them what they are - family. No need to complicate things with all those fancy terms.
I call them my uncles
Nieces.
Chachi