Question: all these straight friends know you're gay? Because I think that even the most stupid straight friend can "understand" that his gay friend may find him attractive… then, at this point, the only thing is "will let me do it?"... just ask, maybe not suddenly... let him fully understand that you are attracted and that you don't seek for his love, but only to satisfy him... probably he has to think he's in charge, that if you blow him that not make him gay, etc... to last if he's not sure try challenge him (I'm so good I will suck you better than every woman you tried...).
However I suggest you to follow the others... try only if you are sure your friendship is strong enough... these things can change your relationship drastically... it can go well but it can also go very bad... maybe in that moment he can enjoy, but regret after...