Ideally, I'd take my son and his friend shopping somewhere that they sell boy accessories and see if there is something there. Like those leather thongs or whatever.
This sounds like a great idea! I've never seen a boy wear a bff necklace - it's really a girl thing (like holding hands, dotting i's with a heart...). Girls tend to couple, boys tend to pack. I do like the idea of them both having the same item, though.