My first thought is that Apollo has an issue with Kitty, and that's what's behind it. That's strengthened by the fact that he doesn't do it at your boyfriend's house.
And, honestly, that's probably it. Does your bf have a cat? If not, he may just be best as an only cat, since he was not raised indoors, and expresses himself inappropriately. If he does have a cat, perhaps they just ruffle his feathers less.
Who's the top cat, Apollo or Kitty?
However, that said, I've seen something played out again and again. When an older cat starts having kidney issues, it can throw the other cats in the house for a loop.
Has she increased her drinking? Is her urine strong/darker than it was?
It's probably not that, but it's always good to use the other cats in a household to give you a clue as to what's going on with a given cat.
So, your dog hasn't made anything worse? Even the friendliest of dogs can throw cats into a tizzy, especially former strays, as they tend to spend their time keeping away from dogs...with good reason!
Honestly, Apollo's issues sound like they will not be solved by regular household measures. And, marking is not something that people want to have their cat doing in their house...especially when they might be the next target!
You can have him checked for a UTI, but it's probably not the case. I would watch Kitty for one, though, or a kidney issue, as I mentioned earlier.
Would you, and your bf, be open to having Apollo stay at his house all of the time?
Marking can also mean that one or the other cats feels stress, and one of the worst things for cats is stress.
Kitty might be stressed because Apollo is being aggressive in a cat sort of way. Or, she could be the top cat, compelling him to compete, and while it looks aggressive, he could actually be on edge all the time. It's difficult to tell without seeing them.
I'd like to add that, one of the more dangerous things if male cats feel stress on a regular basis, is a urinary tract blockage.
Always keep an eye open for not urinating, a bulging belly, etc. Especially if he's got an interest in dog food. We lost a cat, due to his interest in dog food.
Anyway, I'm sorry that I couldn't be of more help.
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