You need to decide for yourself about the commonality of various creatures in your world.
If every dungeon has a rust monster - or even just one in four or five in that part of the world, then it is likely that he has heard of such creatures (albeit with some misinformation mixed in). If rust monsters are so rare that this party is the first to come upon one in that area in the last hundred years, then he has not heard of them. If it falls somewhere inbetween, let him roll an Int check to see if he has heard anything, say at DC 15 or such.
If he fails, tell him the creature before him appears to be a dark red-brown cross between a lizard and a roach, shuffling along the ground, perhaps nibbling at some detris on the floor. If you really want to give him a second chance, allow the party to roll Spot checks (DC 20, as so little is left of it) to notice that the creature seems to be gnawing on minute bits of old metal.
If he succeeds in his Int check, tell him it is an uncommon creature about which he has only heard a few strange tales. According to these stories this creature consumes iron based metals, although some tales speak of consumption of bronze, tin, or copper instead. Even though it will involve player knowledge rather than character knowledge, once he hears this he will know what he is facing.
Note also that - as another poster suggested, perhaps it takes multiple touches to reduce metal to ruin. So his shield is touched - and is notably rusted / weakened. He pulls back. He can get the shield repaired (albeit at a cost, sense it is masterwork at least), but it is not yet ruined. If you wish to explain this seeming change in the nature of rust monsters from the norm, perhaps you could say that it seems a bit smaller than normal - a young of their kind that is not yet able to fully ruin a metal item upon a touch.
