Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
It sounds like he may have constipation with secondary nausea causing gut sounds/appetite loss. Of course, just since he is a male cat, makes sure he can pass urine OK (urinary blockage is a male cat emergency). Now we do need to tread with care. If there is any chance he has eaten something non-edible, has belly pain or vomiting; then we'd want him seen urgently.
Otherwise, for constipation, there are a few home care treatments we can try. To start, you can offer a cup of cow's milk. The lactose in milk can be helpful at increasing gut movement to get stools passed. Another option would be OTC cat hairball medication (ie. Catalax, Laxatone, etc) as it works to lubricate the gut to facilitate the movement of gut contents (hard feces in this case) out of the rectum.
Furthermore, we can also try adding fiber (ie canned pumpkin or a 1/4 teaspoon of unflavored Metamucil mixed into a bit of canned food) to his diet. Just like people, these can restore fecal output regularity. We offer these with wet food to ease him eating of it, while making sure we are getting water into him (as canned food is 35% water). I would also encourage him to drink as constipation can be complicated by dehydration. Make sure he has fresh water and you can even offer low sodium chicken broth if he won’t drink.
While you are doing this, I would advise that you monitor Marmite's urine and fecal output. Though if we use the above measures, but if you aren't seeing feces in the next 12-24 hours, or he begins to vomit, show belly pain, or worsens, then they should be evaluated by a vet. Severe impactions of feces sometimes won’t respond to our gentle colon cleaning treatments, and those cases can require more aggressive treatment (ie enemas under sedation).
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