Montressor is suggesting that they turn back to allay suspicion.
Montresor is very clever. He knows that Fortunato is drunk, and not really aware of what is going on. It is carnival, and his guard is down. He continually tries to trick Fortunato into getting off his guard. This is why he pretends to be worried about his health and offers to turn back.
“[Your] health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible. Besides, there is Luchesi—”
Montresor also tells a joke, pretending that he is a Mason, to make Fortunato think that he can trust him.
You are not of the masons.”
“Yes, yes,” I said; “yes, yes.”
“You? Impossible! A mason?”
“A mason,” I replied.
“A sign,” he said, “a sign.”
He is not a Mason, and he is not a member of any secret society. He is using this an excuse to have a trowel. That way, Fortunato will suspect nothing. The joke about being a Mason may make Fortunato think that he is one, and make him believe that he can trust Montresor, and his inquiries into his health are actually genuine.
“The nitre!” I said; “see, it increases. It hangs like moss upon the vaults. We are below the river's bed. The drops of moisture trickle among the bones. Come, we will go back ere it is too late. Your cough—”
Montresor is convinced that if you do not get away with murder, you are not really getting revenge. This is one of the reasons he is bringing Fortunato down into the crypt in the first place. He knows that once he bricks him up, no one will ever find him.
The constant references to going back must work. Fortunato is no more the wiser. He is so obsessed with seeing the wine, and so drunk, that he trusts Montresor. He is convinced by Montresor's claims about his health and the joke about the Masons, and doesn't seem spooked by the crypt. In the end, he doesn't realize something is wrong until he is actually bricked in.
In this story, Poe shows us the limits of human observation. Montresor claims that Fortunato did him a "thousand" injuries, but chances are they were small or imagined. The reality is that there are plenty of madmen among us, and sometimes we just have to be aware of our surroundings and the people we hang out with and what their true motives are. People manipulate us all of the time. The results are not always as deadly as they were here. We should remember though, that Poe was inspired by a true story when he wrote this!