It's definitely not territorial fighting. The older cat had a sister - I took them both on at the same time - and after a couple of years of them living together in bliss, she had to spend a week at the vets. When she came home, they were mortal enemies - largely at her instigation. The fights were absolutely horrific, and I did separate them, for about 18 months. Every time I tried to reintroduce them to each other - verrrrrry slowly, calmly, as per all the guidance - they attacked each other.
This gave me a horrific insight into what territorial cat behaviour looks like - both the immediate violence of it, and the slow burn months after they had finally been reintroduced, when they begrudgingly accepted that the other one lived here, but if he entered the room, she would leave and vice versa, they wouldn't eat in the same place, they chose different litter trays as 'theirs'.
It took years for them to love each other again - properly love each other, sleep together, move around as one unit again - and when she died, he was heartbroken and would call for her/hunt the house looking for her.
Hence the introduction of the new cat.
They hit it off straight away, after about 3 weeks of being kept separate and scent swapping/territory swapping. Once they were happy to settle in each other's halves of the house without sniffing every inch of it to try and find whoever made that smell - when I territory swapped and they just curled up and slept - I let them meet, and they were instant best buds. The younger cat was a kitten, and he just wanted to be with his old buddy, and the older cat started playing again, and settled at night without crying for his sister.
I thought I had won. And I had, for ages. Well over a year.
The younger cat is asserting his dominance, but not in an inappropriate way, other than he can't seem to moderate his strength. He does have a neurological muscle weakness and I'm starting to wonder if it's that - that he didn't learn how to do it as a kitten, because he had no strength as a kitten. He's only just learned how to jump and climb.
They don't fight all the time, or half the time, or even 5% of the time. And when they do, it's either playing or just a response to being groomed at the wrong time.
We have three litter trays and they both use the same one. They have separate feeding stations but migrate between the two of them as a pair. They are happy to share, and share space, but the little one is a dick with his teeth.
I haven't used Feliway for ages, so that's a good shout. I've also ordered a surgical jumper for the older cat just to give him time to heal - hopefully the two in combination will mean things calm down.