Definitely get Epsom and if you can medications. He either constipated , but i think also could have internal infection. Let us know when you get the meds. Also you can do daily 50 % water changes if you can and full water changes like you usually do at the end of the wk. Take the decorations out , if you might already do it, sorry i don't remember. So take the decor out and the gravel out so you can monitor the poo.
Epsom you will need to pre mix and make sure its dissolved before you put it in the tank. Pre mix first day 1 tsp/gall. We will increase it later.