Your son is your uncle's grandnephew. Your uncle is your son's great uncle.
Your uncle's son is your first cousin. Your uncles grandson (son of his son) is your first cousin, once removed.
The son of your uncle would be your first cousin. A child of one of your parent's siblings (your uncle or aunt) would be 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
Your uncle would be your sons great uncle. Normally your Son would also call him 'Uncle .... '
I call them my uncles