Why is my cat missing the litter box?

Make sure the box is cleaned often. Not just changing the litter and removing the solids, but actually washing the box. If it's not clean they won't use it. Sprinkle a little baking soda in the bottom before you add the litter.

Have you changed litter brands lately? If it's been a year it has nothing to do with the other cat being there and then leaving. Have you moved the box? Or are you suddenly leaving him alone at home a lot? My cat used to punish me by peeing or pooing outside the box when he felt I was away too much. A lot of cats react badly to changes in their routine or placement of things like food bowls or litter boxes.

One obvious reason could be that the box isn't clean enough, or that it's in a location where the cat doesn't feel safe (maybe it's in a corner where they feel trapped, for example.) You have to scoop at least once a day, sometimes more. Cats are very clean animals. They won't want to go in the box if it's dirty and they don't want to step on feces.

Some cats hike their butts up too high to keep it inside the box.

try to introduce a lidded box without a door and soak the area with biological washing liquid is he neutered? i know boys can still spray...xx

Higher sides on the box so he can't hang his butt out.

Your mom's cat was marking territory, so now he is.

I would get a box with higher sides, or a box with a hood to catch everything.