I was reading this book by an animal behaviorist, "The Cat Who Cried For Help," and there's a cat in the book who pees on his owner and all her things. The behaviorist says that this is definitely stress focused around the owner. When he probed further, he found that the woman had had her new boyfriend move in exactly when the cat's behavior started... because of the intruder, the cat was marking the woman as belonging to her.
Sounds like something similar with your cat. He is very attached to you and may have been suffering from things like separation anxiety whenever you left... now that the new cat is there, he's even more worried about showing that you are his.
In the case study in the book, they used the anti-anxiety medication buspirone and some behavioral training to solve the cat's problem. The medication worked right away, but then they did things like have the boyfriend take over feeding the cat, so the cat would associate good things with the boyfriend. After a few months, they tapered off the medication and the cat was fine.
In your case, maybe you could give the cat treats only when he's near the new kitten, so he associates the kitten with good things.