If you both are that close I would ask him for lunch or something and tell him that you need to tell him about something (or something in that matter) and let him no how you feel if not for him at least do it for your self so you will never have to think about what if forever . :-)
You need to tell him how you feel. If he is your best friend he should already have a clue if he really knows you. If you can't tell him in person write him a letter. Thats always easier and you can get out all of your feelings. You can't live like that. You just get your heart broke everyday like that. Tell him then it will be for him to decide and if nothing comes of you and him, atleast he will stop talking so much about his fiance. Good luck.
I think if he is already engaged, you should respect them as a couple and not try and throw a wrench into the works. You need to consider how his fiance feels. Would you want the stress of someone else going after your fiance and ruining your plans with someone? I don't know.... I just believe that, what you do to others, you are doing to yourself. Everything in this universe is connected. If it was meant to be, the both of you would be together before he got engaged. Things rarely work out when they are forced.
I would tell him, if I were in love with him. Also you would never know what well happen what if he loves you but he does not what to tell you. Tell him, if you don't he'll be on your mind 24/7 lol
Go watch my best friends wedding... Or tell him how you feel but be prepared for him to say he is not in love with you in that way....I'm sorry life sucks.