First of all, Fortunato is probably already intoxicated since it is the Carnival. Then, supposedly in order to help ward off the cold, Montesor offers Fortunato a bottle of wine, a Medoc. This is a strong wine, and Fortunato's eyes sparkle after he imbibes it; Montesor plays to the foolishness of Fortunato.
When Montesor later continues his feigned concern "Your cough--" Montesor replies,"It is nothing...let us go on. But first, another draft of the Medoc. Quite inebriated, Fortunato is more than willing to take a bottle on his own as he is unwittingly lead farther into the crypt.