I think the smart thing to do is to figure out what you want out of it. Since he's your best friend, you obviously know him pretty well. The thing is, you don't know anything about this side of him. It's a little tricky to the point where you need to be careful and really communicate to him.
What exactly is he looking for? If you have feelings for him and he's looking to just mess around with nothing serious in mind then I would suggest for you to not go for it. If he needs to experiment and have his fun with guys then let him free.
Just be honest and tell him that you would love to go on a date with him sometime but only if he's looking to see if it would go anywhere. You shouldn't fool around if you have feelings for him in case it's truly one sided. Who knows, maybe the date doesn't go well and you guys just aren't compatible, at least you tried. That doesn't mean you aren't sexually attracted to each other. You can then proceed to a fwb knowing a relationship wasn't going to work. This way you would be spared from a full on heartache.
There is also a chance you guys do work out and end up together in a relationship. You never know until you try. You just have to take it slow and be smart about it because your friendship is what is important and should be put first.
If I were you and my best friend told me he was bi and wanted to go on a date, I totally would only after opening up to him about my concerns. I ain't no side bitch lol. You shouldn't be either. Just remember to keep your emotions in check if things don't go your way. He's still your best friend and you still want the best for him.