You have a straight guy friend, however for the longest time he thought you were straight too. But you can't hide your feelings anymore. You want to expose yourself to him. I know it is quite heart pounding but still, you can do it with the proper way of introducing your gay self to straight guy.
As much as you wanted to be honest as soon as possible, it might back fire. You need to do it slowly and you need to prepare your friend for a revelation that can make or break your relationship.
So you are watching his favorite sitcom with pizza and beer on the side, then a commercial break. This can be a perfect timing to open up a topic such as the gay world. You are not going to reveal yourself ok? You just want to expose him to a world which is far from his' and make sure that you are telling good things about it. Start by telling how fun it is to be gay with all the perks in clubs or by telling him that gays are more than what other people think they are...
Once in a while bring up that topic. I am sure he will get a hint. If you feel that he is fine with it then sooner or later you can introduce the gay you to your straight guy friend. You have to remember, if he is a real friend then he will have to accept you otherwise it's not your loss. Maybe he will react unexpectedly but if he realized that you can't do any harm then he might be the one to apologize. After all you were able to tell him that you have no other intentions.. for now!