I'm sorry to hear that Kitty isn't feeling well refusing to eat and is not defecating.
If he isn't eating he won't have much ingesta to make stools. Conversely if he is unable to pass stools then he will have a back up of ingesta and will feel full, not wanting to eat.
So it is important to think about whether not passing stools came before or after the loss of appetite, and what his last stools were like. Were they very small or very large and hard and difficult to pass requiring lots of straining? If his stools were pretty normal, but just became less as he ate less then his primary issue is loss of appetite.
Changes in appetite can be related to eating something he should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm disease, hyperthyroidism (a tumor of the thyroid gland) with internal organ failure, primary internal organ failure, or even infiltrative cancers such as lymphoma.
Because he hasn't eaten normally in several days I am very concerned about him. If he isn't drinking well now either he will quickly become dehydrated, especially because he is eating poorly as well, and as his liver breaks down fats for energy to live he may develop a type of liver disease called hepatic lipidosis. This happens when the liver becomes overwhelmed breaking down fats for energy and is unable to perform other normal functions.
Ideally he would see a veterinarian today since this has been going on for more than 72 hours. Simple stomach upset should pass within 24 to 48 hours. They could examine him, run blood tests and possibly check radiographs and/or an ultrasound to evaluate him and know best how to treat him.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate him.
If you cannot have him seen for whatever reason there are some things you can try at home.
At home to try and settle his stomach you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and may help him feel less nauseous so that he will eat and hopefully start passing stools again. They are quite safe and can be used long term if necessary.
You can use a medicine syringe to try and force water into him orally.
A couple of hours after giving the acid reducer I recommend offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow. If he eats this well you can add 1 tablespoon of canned pumpkin (not pie filling, just pumpkin) to stimulate intestinal motility.
If he continues to not eat he should see a veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.