I feel like you have some fear of trusting your feelings.
Something is going on in a biological level. Let's just say your sexual hormones are dispersing in his direction, and his are doing the same in your direction. You smell each other's pheromones which is causing your attraction. You both probably have boners and don't know it, and are afraid to act.
He is trying to respect your heterosexuality, but you're leaning a bit to his side, and it scares you.
I wish I could make you understand that what you're experiencing is something beautiful.
Don't be afraid to go on his side for awhile. You can be gay behind closed doors for him, and he should be able to respect you when outside those closed doors considering he knows you "better than mum".
He probably wants to know you inside and out. You both know each other on the inside, now you want to learn each other's outsides.
If you open up to him or make a move, and he doesn't return the feelings, IT IS NOT THE END OF THE WORLD! He is gay, and should understand your feelings, but just because he may not share those same feelings for you doesn't mean he'll look at you funny. He loves you as a friend, and do you seriously think something could change that?
He senses that you're in distress over this, but you won't tell him what's bothering you. It's going to drive him crazy if you don't tell him, and may very well end the friendship.
You're afraid sharing your feelings with him will end the friendship, but really remaining on a limb like this is can end it. He may very well feel the same for you, and you don't know it. Either way, telling him will make you even closer friends.
Inbox me if you'd like to talk further about this, ask me questions, or vent off some steam. You've taken a big step by getting on a gay forum to ask for help with this, and I commend you for that. You're securing your "masculinity".