Thank you again Susan,
Now again considering all William is telling us, he is only saying that he feels unwell and rubbish at the moment. Therefore, we have to start piecing the small clues we have to try and rule out and consider possible causes for today's sudden lethargy.
First, if he has a bit of ocular discharge, this could be a clue to a brewing cat flu but without other sigs, we cannot rule out this just being an incidental findings. Otherwise, if he has had issues with diarrhea, we do have to be a bit concerned about any underlying GI disease. That said, the character of his eating does make concerns about nausea inducing conditions (ie pancreatitis, gastroenteritis, etc) less likely since he is willing to eat but only less inclined to get up and do so.
Now those aside, there are some points in your history do make me concerned that there may be a more chronic issue afoot. First, if we have an elderly cat that passes a large volume of urine even when he hasn't been eating/drinking as usual and therefore has reduced intake; that does make me a wee bit concerned about more subtle underlying issues like dysfunction in the kidney, heart, thyroid or liver. And while losing a companion is difficult for cats, unless he has had a poor appetite for the past year, this wouldn't likely cause our weight loss. Instead, I'd be concerned whether this is also a subtle hint of a chronic issue that would be siphoning weight off of him despite his eating well up until now.
Overall, your lad isn't giving many clues to what has suddeny caused his decline today. But with the other issues of weight loss over a year and possible increased urinary volume; I'd be a wee bit worried that he has a more chronic issue that is perhaps coming to a head here. Therefore, we'd want to montior him closely just now, encourage him to eat and drink.
Favorite foods are allowed or you can tempt him with a light/easily digestible diet. Examples of this would be boiled chicken, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used here (ie Hill’s I/D or Royal Canin’s sensitivity.) These all tend to be well tolerated by cats with sensitive GI's.
Further to this, if he really starts eating poorly, then we do have to consider syringe feeds to get food into him. In that case, you may want to try Hill's A/D (LINK) from your local vet. This is a critical care diet that is comes as a soft, palatable pate. It is calorically dense, so a little goes a long way nutrition-wise and this could just help get some more calories into him even if we can’t get a huge volume of food in. As well, for syringing food, you can use the animal version of Ensure (balanced for animals dietary requirements) called Clinicare Canine/Feline Liquid Diet (LINK) . It is actually by the same people who make Ensure, but is formulated to meet out pet's dietary needs. Your vet should be able to order it for you but it is available without a prescription. They also make one specifically for older cats with kidney troubles, and this could be an alternative for an older cat. This way it would a means of getting food and energy into him to help him fight this underlying issue.
On top off all of this, you do need to keep an eye on his water intake and hydration. To check his hydration status to make sure he is not becoming dehydrated (a concern if he is urinating more) there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether he has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this (HERE). They use a big dog but it makes it easier to see and the principles are exactly the same. If you are seeing any signs of dehydration already, then you do want to have your kitty seen by his vet before this gets out of control for him.
In regards ***** ***** you can do to help stave off dehydration at home (though do note that if he is already then he will likely need more the oral rehydration), encourage him to drink but offering fresh water or even low-sodium chicken broth. As well, wet foods (as mentioned above) are 35% water, so getting him to eat will help us deal with water intake as well. If he isn't amenable to drinking, you may wish to offer unflavored pedialyte via syringe feeding. While we cannot do this if they are vomiting, it may be an option for this situation. A typical maintenance rate for hydration in an animal 48mls per kilogram of his body weight a day. If you do give syringe pedialyte, this should obviously be divided up into multiple offerings through the day rather then all at once. This value will give you the total he needs for the day and is a good starting point to give you an idea of his daily requirement. If he does vomits if you give pedialyte, I would discontinue this as a therapy. (since we don’t want him vomiting because of our intervention).
Still on top of this, it'd really be ideal to have him checked over and a blood +/- urine sample tested. This can tell you if there is a subtle underlying organ or metabolic/hormonal issue afoot that is just starting to tap his energy reserves on top of his weight. Depending on the findings of that, it may be a case of supportive care to address one of these issues or if his white cells are found elevated, prophylactic antibiotic treatment to tackle any opportunisitic infection bombarding his elderly and weak immune system.
Please take care,