That's pretty much what a plecostomus does; eat and poop. They tend to be nocturnal, and he would probably like a cave to hang out in. Some fish take a while to come out, so you should see him more in a week or so.
I hope you have a big tank. If he is a common plecostomus (pleco for short, some people write it pl*co), he will get to 12" in no time at all. If you have a tank that's less than about 75 gallons, and he is a common pl*co, I would consider trying to take him back to the store for something that doesn't get as big