Introducing a new family member is difficult for any cat. They don't like change. Best thing you can do is keep the new kitten apart for a while, like giving them their own room for a few weeks. The older cat should feel like it's 'their' home, new blood upsets the balance of power.
After a while you can reintroduce the kitten. When you pet the new cat, use the same hand to pet the older one. Cats do a lot by smell so mixing their distinct smells will make it easier.
Also make sure the older cat has their own food, their own litterbox and make sure the kitten doesn't take in any of their favourite spots to sleep.
It's hard work, you'd have to constantly pay attention to the lil guy and you can't control what happens when you sleep.
Also kittens need to sleep 22 hours a day but with an older cat around they're gonna wanna harass the older cat to show they mean business.
I would not recommend introducing a new cat to any cat over 2 years of age. At this point best thing you can do is wait it out and hope your old guy improves.
It's important to understand why your cat is not taking to the new one. It's primarily the change in a familiar situation.