Male cat urinating everywhere.

[Brat]

For the past few days, we've encountered a problem with my oldest male cat.
He is 9 months old, I have two other kittens that are 5 months old. (They are all brothers from two separate litters) They love eachother, play all the time..etc.

Last week, I changed the type of litter from clumping to pine. Unfortunately I did not make the switch gradually which I definitely should have. The two kittens have no problem with it. The 9 month old uses it as well, he poos in it. But for the past few days he's been peeing all over my house. I caught him once positioning himself on my curtains as I watched him. I thought he was sitting down to look at me. Little did I know he was squatting to take a nice looooong pee on my curtain. I jumped to him as soon as I smelled it, cleaned it up. It reaked for DAYS in my office room. Then my boyfriend and I started to notice the smell in the living room. It seems he peed in my fake plant. After cleaning that up, there was still a horrible stench and we couldn't find it! Finally today we found it.. he had peed all over a pillow we had.

I've only seen him do it once. I'm not sure if this means he is starting to mark his territory or he's spraying or he hates the new litter. I saw a male cat spray today and it's not what my boy does.

Also, about two weeks ago, we switched foods but really it started when we switched litters I think.

We have made a vet appointment this week and I think I'm going to set up another litter box with our old litter. Overall, he seems healthy and active as usual. Bouncing around, running after the little ones, etc.

My main question I guess is, is he marking his territory? Does he maybe not like the litter? (weird since he does poo and I believe sometimes pees in it)

Our apartment and clothes stink! BLECK!