The trick is, keep it neutral, short and sweet.
If there is another BM speech after yours, then you can keep yours to early days.
It doesn't have to be shared experience, although it does help - it can be any funny incidents that have happened in childhood.
Remember, you are not there to humiliate the groom, or god forbid the bride, or anyone else there, so if you are not a professional comedian, don't try and make your speech an audition for a stand-up gig!
Just be nice about your friend, how cool he was as a kid, and how cool he now is as an adult - and they will hear more about that in a minute - you don't have to go on for hours.
Good luck.