Out of expirence you should tell him how you feel..
BUT you obviously value your friendship a lot and If you don't want to risk losing a brillaint friend I would support what he wants to do.
If you do decide to tell him how you feel you should first find out how he feels about this girl and ask him while he is sober if he actually is Bi.
Again I can see you really like him and you should tell somebody, I know you told you mate but she was not supportive at all. Text someone you trust and will give proper advise.
Although I do recommend you tell him.. it can go either way... but suffering when he is around other girls is not the way for it, it will start to hurt more and may damage your friendship. Everything out in the open is always good.. if he feels the same, then you will be over the moon. If the feelings aren't mutual then you know and can
try and get over him, with his support.
I hope this helps..
