The person described as "him who clamoured" is, of course, Fortunato. It would seem that Poe chose to avoid naming his victim by name in that particular paragraph because he wanted to create the impression that Montresor could only make out a vague form in the dim light. Montresor has built the stone wall to such a height that his flambeaux only "threw a few feeble rays upon the figure within." Those are the last words of the paragraph preceding the one in which he describes that figure as "him who clamoured" and as "the clamourer." In that paragraph he also describes his captive as "the chained form." The only purpose seems to be to create a noir picture of a shadowy figure and not to avoid mentioning Fortunato's name for some obscure reason. Montresor is not afraid to mention Fortunato's name. He does so in the very next paragraph, in which he says:
But now there came from out the niche a low laugh that erected the hairs upon my head. It was succeeded by a sad voice, which I had difficulty in recognizing as that of the noble Fortunato.
In fact, he twice calls to Fortunato by name before the story ends.
No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells.
Fortunato has given up clamoring and has given up pleading. He is still alive, but he is totally defeated and is without hope.
It would seem that if Montresor used Fortunato's name in the paragraph in question, rather than calling him "the clamourer" and "the chained form," it would only seem to make the interior of the walled-in niche lighter. Poe must have wanted to emphasize that, in addition to everything else Fortunato was going to have to suffer, he would be all alone in the pitch darkness. In effect, he would be buried alive.