If I were you, I'd get out in front of this and let the kiddo's mom know what happened, and what's going on at home. Wouldn't you want to know if YOUR child were put in a dangerous situation? One that is not unlikely to recur, given his father's drinking?
Lying is a knee-jerk protective mechanism for alcoholics. It protects their addiction and protects them from the consequences (or so they hope). It's not a matter of delusion in that instance. It's a conscious decision.
If I were asked to name ONE characteristic common to alcoholics, I'd choose lying. And if they aren't lying to others, they are lying to themselves.