well if you socialize enough you will understand how important the "bro" in conversation with some stranger that for sure you don't even know his name..
of course you can use mister and sir but it will be so awkward to be so formal in casual situation..
but yeah if your friends calling each other "bro" everytime, he must be beruk that dont know how to call someone name..

but better use "bro" than "der".. it is so mat rempit style..