The answer is, his "superego" has influenced his choice to abstain from the drinking at the party.
The superego is the moral segment of the identity and gives the ethical principles by which the ego works. The superego's reactions, forbiddances, and restraints shape a man's inner voice, and its positive desires and beliefs speak to one's admired mental self portrait, or "ego ideal”. Like in the given case Dawud is inclined towards intoxication but his superego which makes his moral and ethical component forbids this indulgence.