Pooping issues

My cat is around 1 and is strictly indoors. Recently he started pooping outside his box, like 75% of the time. I thought at first maybe since it's getting warmer out, he finds it too hot inside his covered box. But even when my apt is cool, he still does it. And I have to keep the lid on, otherwise he'll see the wires behind it and try to eat them.

Shortly after getting off the phone with his vet, he wiped his bottom across my bare mattress and left skid marks. The fur surrounding his bottom is dirty with poop and apart from right now he hasn't kept the area nearly as clean as he used to.

He's a domestic neutered shorthair, who stays indoors as I pointed out.

Cats usually avoid the litter box for two reasons, they don't like the box, or the litter is not right. I would try a tall sided open tote with one side cut down to see if that helps. That would still hide the wires too, but would be open.  First change the litter, he might not like it for some reason. If you can trim his hair on his bottom when he is laying down, that might help, it sounds like he may have some too loose stool going on there too. You don't give him cows milk do you, that always seems to cause diarrhea. Could something be bothering him when he is in his box? Like a washing machine  or something else noisy? Good luck, I hope you find a solution, there is nothing worse then having to smell cat poop!

He's like obsessed with his litter, I'm too scared to change it. He had issues with an open box when I first got him. His vet recommended I bring in a stool sample and bring him in so they can shave his bum, and see if he needs his glands squeezed. The vet said he can pick up parasites from bugs and he has killed and eaten a fly or two and a moth since I rescued him. He has also killed a cockroach but didn't eat it.

Just a thought, maybe the litter box is too small now? Has he grown since you got him? Some "kitten" litter boxes are very small.

His box is for a full sized cat. I took his vets advice and had his glands squeezed. They were fine, no sign of blood or infection. I also sent in a fecal for them. He is growing into a big cat. 13lbs 9oz and he's still growing and not overweight.