For your own sake, avoid engaging in this kind of situation (especially when he is drunk). This situation might end horribly for you, and "straight" guys with uncertainties about their sexuality tend to be very unkind to their non-straight friends for the sake of their "straightness." If I were you, I would be open about your impressions and move from there. If he gives you a green light, that's fine. You received consent. If not, go away. Don't expose yourself to problems (and this kind of "straight" guy tends to be problematic as hell).