Don't say anything to your brother- people make some odd decisions when putting their weddings together. You have been given an honor even if you're not going to be his Best Man- that's not nothing either. You don't know how or why this was done, and he may or may not look back later and think it should have been you- but it really doesn't matter- you WILL be up there beside him, and you WILL always be his brother. Nothing good will come of your having a tantrum and stalking off now.
As you get older, you will see people make decisions they don't mean to, or just really shouldn't make. There is nothing you can or SHOULD do about it. The mature, steady thing to do is to smile, make sure you look out for him no matter what happens- his friend may or may not be his friend in 5 years, but you will always be his brother. Young people make decisions based upon what is closest to them when they're making them, and then find out later. It's called growing up- so don't make a mistake about this yourself. You will regret it.