The water change could of stressed him out, what I would recommend is moving his tank to a dark room that's very quiet, the next day it should make him back to normal, I would add a bit of AQ salt too.
If it was from the water change, gently scoop him up with a cup & don't stress him out, if does stress out apply stress coat, or your water conditioner if it has a stress coat forumla like "Aqua +" it has a stress coat forumla, protects scales & it's a water conditioner. When you change his water, don't plop him him make sure you have enough of his old water. Pour him in a zip-lock bag after your done his water change, if he's in a cup try to see if you can float the cup, if you can't put the cup in the bag [with him in it] and move the cup to the corner and pour so he is in the corner & has enough water, then float him for 10 minutes, after 10 minutes scoop the tank water & pour it in the bag, and then let him sit for 15 minutes then release him, if he lays at the bottom AFTER the water change add AQ salt then make sure he is in a dark and quiet room, if he isn't bloated or you didn't feed him much try crushing up a pellet and giving him half of it if he doesn't want it, take it out. Then leave the room & let him relax & let the room be quiet.