Your uncle would simply be a great uncle to your son.
He would be their great uncle.
He would be a great uncle to your son.
By your uncle, you must mean your father's or mother's brother. In some societies one's father's or mother's brothers are considered as grandparents to ones kids. In that way your children can call your uncle as Grandpa or Grandfather. They can also call him Uncle by their relation to him.
He would be his great uncle and his kids who be your son's second cousins.
The kids of your uncle are not your childs second cousins.... They are your childs first cousins once removed as they share both grandparents and are removed by one generation
He would be a grand father to your son.