Need help with territorial cat

I got a second cat, a female, so that my cat would have a pal, but my male cat has been very territorial: fighting, his poop smells much worse and he won’t cover it in the litter box. The worst part is the smell. It wakes me up out of my sleep. He’s been to the vet and got a clean bill of health. We introduced them slowly, fed them separately for weeks, provided equal beds and scratchers. Nothing has helped. My house smells like death whenever he uses the litter box. I don’t want to rehome him but I’m out of ideas.