A brother of either of your parents is your uncle (it's not just a brother of your mum.) I know in some Asian languages there are many different words for family members depending on whether they are older or younger etc but in English there is no such distinction.

A brother of your mother is called your uncle. There is no specific word for one that is younger/older.

As above, he is your uncle. However you would address him as 'Uncle First name' or commonly just by his name. Apart from parents and grandparents Eng, ish speakers tend to address family members quite informally.