Anxiety between cats?

[kosame]

Where do I begin...
I have two kitties, Theo and Remi. They used to love each other! They were partners in crime, always so silly and sweet and the best of friends.
Theo's been having a tough time recently

Remi, his little (adoptive) brother, went to the vet for a neuter and when he came back Theo didn't recognize his smell!
He was so stressed that he completely stopped eating, lost weight and when I was able to get him to eat, his physical state went back to that of a normal, healthy kitty. But he's been so unhappy

For the past four months, he's not himself. Everything makes him anxious, he is often attacking and trying to fight Remi (who used to run away, but now he lashes back), he meows so sadly/paces and he just seems so sad and unhappy. It breaks my heart.
He used to enjoy the company of other cats, but now he hates them and gets so afraid around them. My boyfriend has a kitten, and if he even smells the kitten on Remi (they are good friends and we take them on walks together), Theo loses his mind. Recently, right after smelling bf's kitten on Remi, he has stopped eating again. Whenever anything, no matter how minor, changes in his life, he refuses to eat and vomits on occassion. I was of course, worried sick, so I took him for a vet visit.
At the vet he was a whole different cat. The cat I used to know. Purring, rubbing, rolling on his belly, asking for pets and kisses. We ruled out health troubles (thank goodness) and he's on some meds + I got them Feliway. That was a month ago and honestly I'm not sure if anything is improving.
Sometimes the kitties are fine together and they even groom each other, sometimes they're at each other's throats. But I feel like that's improved a little. But Theo still seems generally unhappy. Pacing, crying, etc. Not sure what to do. I just want him to be a happy kitty <3
He is back to himself whenever Remi isn't around, but I can't physically keep them apart the majority of the time, with my living situation. Any suggestions would be so much appreciated!

 

[jen]

Sometimes the kitties are fine together and they even groom each other, sometimes they're at each other's throats.

The fact that they groom each other sometimes means things are generally fine. I really think you are misinterpreting rough crazy kitten play for hating or being anxious around each other. It is very common for them to react badly when one cat leaves and goes to a stressful place like the vet, and then returns and fights break out due to not recognizing the smell. You are right there. But since they groom each other, that is no longer the issue. Pacing and meowing is a cat thing, that doesn't make him unhappy. An unhappy cat would be hiding in fear all the time.

Now, we need to focus on the not eating which is another thing altogether. What all did the vet do to rule out health issues and what meds are he on currently? Have you tried different varieties of foods? Canned and dry?