I’m in the same boat man, my best friend just got engaged and I also I have to give a best man speech. Think of it this way: he knows you stutter and is confident in you enough to the point where he would ask you to give a speech, that’s some kind of friendship right there. He’s gonna love it no matter what. Just keep it short, simple, and light hearted. The more you laugh the more the audience will and it’ll be better than you think it will be. You got this, definitely prepare something and practice it more than you’ve practiced anything. that’s what I plan to do hope it helps. Good luck!