your uncle is your sons great uncle
You could probably contact him on the phone, or via skype if you are tech savvy. If you mean the name of the relation between yourself and your maternal uncle's son, that would be a cousin (first cousin to be exact).
it great nephew :)
The father's son is the uncle of the son of the father's daughter.
Your uncle's son is your (first) cousin.
Nephew
You would be his uncle and he would be your nephew.
A man is his sister's son's Uncle. A woman is her sister's son's Aunt .
His great uncle
He would be the teachers cousin. imagine you had an uncle and he had a child . that child would be your cousin .
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your son would be the baby's uncle. Nobody would put such a fine point on the relationship to call your son the "Step-uncle" or "Half-uncle".
Your niece's son is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).