Adult cat refuses to use litter box since I got a new kitten

I found a kitten under a neighbor's car 3 weeks ago and have kept her. So far, my adult cat whines and hisses whenever he sees her and has been asking to go potty outside like a dog. The weather has gotten cold enough that he doesn't really even want to be outside, but he will try to hold his pee rather than just using his litter box.

Each cat has their own litter box, but since I've started transitioning the kitten into free-roaming indoors (before I took her to the vet she was quarantined in the bathroom) she has been using both litter boxes. I scoop multiple times a day, but right now I'm worried my adult cat will give himself a UTI from refusing to pee indoors.