4 weeks later is a bit late for this cat, but for anyone else: If he's super lethargic or has not peed for like eight hours, then he should see an emergent vet. Otherwise you'll want to rule out an obstruction or laceration or something like that, so a regular vet visit and some kind of imaging may be in order. After that, if you still don't know what's wrong, that's when you start playing with diet and supplements. Cats are good at running off and hiding symptoms, so establishing a play-eat-sleep routine that mirrors yours is the best way to identify deviation. Also, it's really hot out so make sure they have lots of water.
The emergency animal hospital is very expensive and requires full payment upon discharge. My dog refused to walk for a month and a half and seemed to be in immense amounts of pain. I brought her to Upstate Vet Specialties and because she was so scared they sedated her and they couldnt tell me what was wrong with her. Most expensive sedated bath and nail clipping in the world at $400 paid in full upon discharge. Google ways to help your cat with constipation, maybe a warm washcloth massage.
Take him to his normal vet. It could be constipation, but only a vet can tell you if it's anything more serious.
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