How to stop a male cat from pooping outside the litterbox?

This is the number one sign of illness in cats. First the vet needs to see the cat and do a urinalysis (not expensive). Inappropriate urination in fixed cats is usually a sign of a urinary tract infection or crystals forming in their urine. Both conditions make it extremely painful for the cat to urinate, so they avoid the regular place of urination as they associate it with the pain and try to find somewhere to go that won't hurt. They often will show no other signs of illness except the urination, but you can watch for small, frequent amounts of urine, blood in urine,or acting like they are going to go but then not doing anything. Both conditions are easily treatable (with antibiotics or special food) unless they are not caught and wreak havoc on the cats system. Once both conditions are ruled out you can attack this from a behavioral standpoint. There are pills the vet can give you that will calm the cat and stop this behavior if it is stress induced. You also need to make sure you are cleaning the stains with a cleaner that says enzyme or enzymatic on it. No other cleaner will destroy the scent of urine (you may not smell it but your cat can) and the cat will continue to go there if they can smell it. Lastly if his hasn't been neutered that needs to be done ASAP, as nothing you do will stop him from marking his territory if he isn't.