My kitten is getting fat

[BelleCat]

He's 8 months old.
He's been getting 'fat' for few months now, but in the last week he about doubled in size.
I got him neutured about 2 and a half months ago, and he started bleeding right after. The vet thought it was Parvo, and said that he tested positive, but I never saw the test, and he didn't show any symptoms prior. Few days after, he was doing much better, so I got him tested again and the test showed a very faint dot, that was barely there.
I took him for general health check and deworming last week, and the vet said that he's just fat, and since then he's only gotten bigger.
I don't see him eating, he doesn't touch the wet food that he used to love, he is acting a bit lethargic and has been hiding. Yesterday he was scratching in the litter box for around 10 minutes before I went and got him.

I did get a dog when he was 2 months old, he loves her food, so that could be what's doing it.

I have 6 cats so I didn't notice if he actually uses the litter box, I might separate them to check.

I feed them royal canin kitten (cause they like it more) and applaws kitten dry food, and applaws wet food twice a day.

Am I being paranoid? I'm going to get all of my pets on raw to be safe, and I want to take him to another vet but I have a feeling he will just say that he's just fat again.

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[Alicewond]

Is he an outdoor cat, could he be feeding at a neighbours? In addition to home?

[BelleCat]

@Alicewond he does go to my backyard, but it's cat proof so he can't get out.

I'm actually thinking he's constipated, but I'm not sure.

[dementedpixie]

I would switch to adult food. They don't need the kitten food so much once they have been neutered

[dementedpixie]

He looks a bit like one of mine. Mine is too heavy too although his brother's weight is fine