Why is my neutered cat nervous and not getting along with senior cat?

Such behavior can be a dominance issue and in older pets may be a sign of changing social dynamics and i would schedule a vet check to rule out any medical causes. The comfort zone diffusers for multicat household or the original feliway pheromone diffuser may help but i would not introduce a new kitten until things settle down

They may always be enemies but there are a few things you can try to help calm them. I would start by putting some feliway diffusers throughout the house. They have a multicat version now. Feliway is a feline pheromone which is helpful to calm cats. It doesn't work on all. You could also consider putting some essential oils throughout the house. Be sure to get ones that are safe for cats. Visit oilyvet.com for more information. Also, make sure you have at least 3 litterboxes throughout the house, if possible, to prevent competition. If all this fails, you could talk to your vet about putting one (the instigator) or both on an antianxiety medicine to help calm them. I hope this helps!

Each cat has their own personality, just like people. You cannot force him to change. If you have access to getting some Feliway diffusers you may want to try that. They send out calming pheromones for cats and it works in most cases to make shy/timid/scared cats more comfortable. ( https://www.feliway.com/gr )

It takes 4-6weeks after neutering before the hormones and related behavior subsides. Be aware however that not all bad behavior is hormonal and some cats will never be friends and many need to be segregated. Calming diets and supplements such as kalm aid can help alongside the feliway

Thanks for using Petco Pet Education Center, formerly Petcoach! On the whole, I’m not worried about this yet. It can take a couple weeks to feel completely normal after a neuter, especially in adult males. If you have pain medication still, go ahead and give it as even mild pain can cause some cats to be a little crankier than usual. Sometimes, a long trip to the vet or anesthesia seems to disturb cat relations and anxiety is more of a problem. If this behavior is not improving with pain medication, or persists for another week or more, consider these anxiety-reducing home helps. -Feliway- calming cat pheromone- comes in many forms from pet stores or online. 

- Adding a litter box. Studies show that the ideal number of litter boxes for a home is #of cats +1 . This is regardless of how clean you keep his current litter box.

 -making sure he has plenty of “high space”. The safest (and therefore least anxiety-inducing) place for a cat is up. If you don’t already, get a cat tree or put up a shelf or two that he can go perch on. Best wishes to you and Gus!

I am so sorry to hear about your kitties! I would recommend getting some Feliway multicat diffusers https://www.feliway.com/us/Products/FELIWAY-MultiCat-Diffuser which are a pheremone releasing product that can help these situations. A calming supplement such as Zylkene can also be helpful. Ensure the cats each have places they can hide/get away if needed as well as adequate numbers of litter boxes (one more than the number of cats in your household), water and food available where they can eat/drink without being disturbed by the other cat. If they are still fighting despite this, some training may be needed. You can read this article by a veterinary behaviorist for details on this: https://drsophiayin.com/blog/entry/feuding_felines/

If the two females cats are not spayed I suggest you do that as soon as possible. That could sure be the reason for the tension. Not only that but if the females are not spayed and are going outside they will get pregnant guaranteed as soon as they go into heat. Usually the first heat cycle is around 6-7 months (so now). I also suggest you try some Feliway products in your home. These are basically "happy cat pheromones" that help to de-stress and relax cats. It works very well. Here is a link to their website. You can buy their products at any large pet store or online. https://www.feliway.com/us#redirected Hope this helps. Best wishes.