Your great uncle's son's son is his grandson. Your great uncle is the brother of one of your grandparents.
Children of siblings are first cousins to each other.
Grandchildren of siblings are second cousins to each other.
Great grand children of siblings are third cousins to each other.
So you and your great uncle's grandson are second cousins to each other.
Your great great uncle's son is your first cousin, twice removed.
Your great uncle is your grandparent's brother. So your great great uncle is your great-grandparent's brother. You then are going down one generation (great great uncle's son) who is your grandparent's first cousin. You then remove the two generational difference between your grandparent and you, making him your first cousin, twice removed.
your cousin
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
It means you have (or had) at least two uncles and at least two great-uncles, and that is all it means.
Your son's children are your grandchildren.
great uncle
His great uncle
Your uncle would be your sons great uncle. Normally your Son would also call him 'Uncle .... '
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your grandfather's son's son could be:you (if you are a boy)your brotheryour first cousin
They share a common Grandparent. That makes them first cousins.
Your nephew's son is your great nephew.