hmm.. it should really be someone whos is close to him and been ther for him, someone who will be helpful and responsible if he has 2 best friends one should stand out a bit more than the other and if anything flip a coin lol! if he has 2 brothers same applies that way no one feels like he picked one over the other atleast thats what i would do, good luck!
My FH has 4 brothers but is pretty close with his one brother so he is the best man. If your fiancee thinks his brother would be offended if he didn't ask him to be his best man that's something he should consider. Family is always there.. friends really come and go. The fact that he can't really 'decide' makes it sound to me like he doesn't really have a 'best friend' that should be his best man over his brother.. so therefore I really think he should ask his brother!
Happy Planning :)