hrmm, i dont think hes constipated...or is he? well, most of the bottom feeders (corydoras, otos, plecos), when they poop, their poo is scraped off as they move through the gravel. it is quite common however to see them pooping in long threads, but i think it is constipated. its nothing to worry about, it will take care of itself, it'll eventually get un-constipated and will be normal, happy, jolly, and poopy soon
(i gotta stop using the word constipated before i break out laughing) hope that helps!