Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.
Your uncle's son is your (first) cousin.
His great uncle
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
your uncle is your sons great uncle
cousinlol
He is your nephew, and you are his uncle.
your son is your brother's nephew, and your brother is your son's uncle
Great uncle / Great nephew
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
He would be the teachers cousin. imagine you had an uncle and he had a child . that child would be your cousin .
You are their great-uncle/aunt.
it great nephew :)