Got A Cat For My Cat - May Be Regretting It

[TheManOfFaith]

So I had a 5 month old kitten who was having separation anxiety issues when I went back to work. I just wanted him to be happy, so I carefully researched and adopted another kitten. Everything was great the first week. They played, wrestled, slept, and cuddled together. Just Monday I caught them literally hugging while they slept. But the last 3 days I've noticed my original kitten has been puffed up most of the day and hides from his new brother. He even shies away when I go to pet him. I have no idea what's happened since this time. I can't even play with them together because he hides (his new brother is very play-dominant so my original kitten ends up just sitting out of the way).

The only reason I got another cat is to make my little boy happy.. but now he doesn't seem happy and I'm wondering if I've done the right thing. I don't want his previously vibrant, excessively happy personality to be dulled by depression and he hasn't had a non-puffed up moment for two whole days (aside from when his brother is out of the room or asleep and he's alone with me). I'm really worried. If he sees the new kitten doing anything recently he pretty much stalks him, fur up and back arched.. I can't even play with just him because that means closing the door on the new kitten, who does nothing but howl if he can't be in the room with us and that distracts my boy and he won't play.
I'm really at a loss. If this behavior is going to continue, it's a real disservice to my cat. He walks around with his tail lowered all the time now. I haven't seen him with his tail up and happy for days and I'm just really upset because I love him and I only wanted to do what was right for him. He seemed so happy just a few days ago but I've been really worried about his sudden shift in behavior.

Does anyone have any ideas on what I can do? He is my primary concern, not the new kitten. I love my new kitten, don't get me wrong (they're a week apart in age btw) but my original kitten comes first. He's the one I adopted another cat for.

Notes: it's been a month since I adopted the new kitten and after 2 days they were acting like best friends. It's been a few days of this new behavior from my original kitten.

I would say his tail position is usually like the attachment

 

[dahli6]

Are both boys neutered?
Around 6 months they start to become territorial. Males will try to dominate and even push each other out.
They will spray on everything. I have seen it even with brothers who are raised together.
In a large group, males will seek out and try to destroy the biggest threat to their territorial rights but this is one on one.
For the sake of harmony and also your possessions, if they aren't neutered, get it done as soon as possible.
At 5 months they played together because that is what kittens do, they see another kid and they play.

 

[elliesvictim]

Cats can be seriously jealous. There is a cat I used to feed in my backyard. If my cat thinks I'm paying too much attention to it she does one of three things.
1 avoids me, runs away.
2 runs around howling and hissing
3 has reclusive body language tail down, lower posture.

Try including her more. Pay more seperate attention and give some more love.

Also agree territorial issue but it sounds like jealousy ATM.

 

[TheManOfFaith]

The attached picture was then on Monday. Since then, the behavior from they grey one has become shy or timid/stalking and the oriental has become seemingly territorial.

Things I remembered: a neighbor was moving and she had a litter box she didn't want to pack. It was bigger than mine and she'd cleaned it/scrubbed it out already, so I took it and replaced the one I had with it. That was Monday night.
And I'd gotten a Feliway when I first got the oriental to help them get along but never used it. Popped that in Sunday night.
Finally, I've been really sick so my sleep and wake schedule have been off, so they've eaten a little later in the morning.

I'm not sure if any of that would impact things. As far as giving the grey more attention I definitely do. He loves being carried so I carry him and coddle him a lot. If he seems scared I'll pick him up and hold him in my lap while I watch tv and he'll pure and snooze. The oriental keeps trying to come up too but I distract him.
I'm also curious if my coddling the grey is making the new (oriental) jealous?

 

[PushPurrCatPaws]

Do you use two litter boxes? It might be a good idea to do so.

 

[TheManOfFaith]

I've started to now. one is near where they eat and one is in the usual spot in my room

 

[maggiedemi]

I hope it works out. I love that photo of them hugging while they sleep.

 

[elliesvictim]

Firstly I'd get rid off the 2nd hand litterbox. The cats are learning to cope with each other so throwing a third cat scent in is probably counterproductive.
Literally boxes aren't expensive so I'd put out a couple of new ones and let them sort out where each want to make thief own regular box.
Try not to ever read every interaction and remember some cats play rough and they like to stalk.
Give it some time. This is like an arranged marriage so it will probably take a lot longer to get used to each other.