He is your uncle, in most families he would be Uncle (Jack, Fred or what ever his first name is).
Your Uncle.
I call my mother mom
Your mother's mother is your grandmother.
brother
You could call them as a brother/sister or as a cousin... If you're asking for yourself then good luck with that family.
In Farsi, you would say "amoo" for mother's brother.
His relationship to you is brother-in-law.
uncle
group of mothers
Your mother's son is your brother or your half brother.
A cousin brother means , the son of the father or mothers , brother or sister.
papu