Hello
@JJ-Jedi and welcome
I am sorry to hear your cat is so poorly. I can understand how worrying it must be for you.
When a cat stops eating it can sometimes be really difficult to get them eating again - I have been there with some of my cats (both past and present). It is as though they give up the will to live. The problem is that not eating for 5 days can result in a nasty illness called liver lipidosis, and this is a matter for urgent vet treatment. When did the vet last see him?
It is now very urgent for you to get your cat eating again, if you are to save him. As he has diarrhoea he should be on a diet of home cooked chicken drumsticks or poached white fish, both served with plenty of the cooking juices. Have you tried him with either of those?
For a sick cat who is not eating it is important to stop presenting them with their usual foods. They have started to associate their normal diet with feeling ill, i.e. they blame the food for how they feel.
Instead go now to your nearest pet store or supermarket and buy a selection of about 5 or 6 different wet cat foods. Then put a teaspoonful of each food, one by one, on a plate and set it near him. Do not feed him dry food, wet food only.
Things you can do to encourage him to eat one of the new foods:
1/grind up a couple of cat treats in a new pepper grinder and sprinkle the powder on top of his wet food.
or
2/ sprinkle a little powdered cat nip on top of his food
3/ buy some Thrive freeze dried cat treats in a strong flavour (such as white fish) and break the cubes into pieces and put on his food
4/ liquidise one of the new wet foods in a blender so he can lap it. (don't add water as wet food has a high percentage of water and will become liquid in the blender)
5/ if he won't accept any of the new foods, try him with a little canned tuna for humans - get the one in spring water.
If he is not drinking a lot to compensate for not eating I am very worried he will become dehydrated. He will also need his electrolytes replaced if he is not drinking much or eating anything. Dehydration will make him feel very ill and left untreated will kill him.
Has he had IV fluids at the vet in the past few days? Have a look at his gums - if they are pale and dry looking then he is dehydrated and you need to get him to the vet today. if the gums are moist and pink then he is not dehydrated.
Also you can lift the skin of his scruff between your thumb and forefinger and then let it go. It should spring back almost like elastic. if it stays in a 'tent' position then he is dehydrated.
I would also prepare yourself for force feeding your cat - in case he won't eat any of the new foods you buy for him today.
For force feeding you need to buy a can of Hills Recovery food from the vet. This is highly nutritious and the cat will not need much of it. You also need some food syringes (pharmacies sell them, or the vet may sell you one). The Hills can be used straight from the can, but once the food has been in the fridge it will harden a bit and you may need to warm a small portion in the microwave for 5 seconds to loosen it up a bit. Don't feed it to the cat until you are sure it has cooled down. (Store the Hills in the fridge a plastic food container with a lid or in a small ceramic or glass bowl with a saucer on top. Do not store it in the tin. )
Fill the syringe, then gently squeeze a little food into the
corner of your cat's mouth, never into the front of his mouth. Only a small amount at a time and always wait for him to swallow before you give any more, otherwise there is a risk he could aspirate (inhale) the food and it will go into his lungs and cause pneumonia.
Here is a good video showing how it is done :
Please get a can of Hills Recovery food from the vet today and if your cat has refused all the new foods you offer him today then you need to start the force feeding by this evening. Note, that he may not be as docile as the cat in the video, and you will need to be careful not to cause him more stress in his current delicate state. He will need small amounts of food about 3 times a day. You can continue to offer the new foods between the syringe feeds and once he is eating independently stop the force feeding.
Good luck, please let us know how things go. Sending many healing thoughts for your cat's recovery.