Just call him by his regular name. It is know different from any other person. He just happened to be the kid of the mother/father your mother/father would be marrying.
Uncle
You would call your husband's younger brother a brother-in-law.
In Malayalam, you call a younger brother "Chettan."
Your father's brother is your uncle.
uncle
uncle
Brother in-law
cousin
aunt
The wife of your brother (younger or older than you) is your sister-in-law.
little brother
Uncle
Younger brother in Hindi is called "ā¤āĨā¤ā¤ž ā¤ā¤žā¤" (chhota bhai).