Poor wee Monty
Based on your history, it does sound like he may have caught cat flu (aka an upper respiratory tract infection). Now when we see cat flu, we must appreciate that there are a a few different agents that can trigger this condition (viral and bacterial) and signs we often see include lethargy, sneezing, congestion, watery eyes or noses, appetite decline, sore throat, voice loss, drooling (as some viruses can cause oral ulcers), coughing, and lethargy. Much like flu for us.
In regards ***** ***** this situation, we want to use supportive care as our mainstay treatment for him. That said, if he is showing any snotty/yellow/green colored eye/nasal discharges, then that would be our cue to follow up with his vet. The reason why we'd do so at that stage is because seeing a change in discharge color (from clear) would tell us there were bacteria present and antibiotics would be indicated.
In regards ***** ***** care, there are a few things you can do to help him. First, if he is sneezing and congested you can take him in the bathroom while you run a hot shower. The steam will help loosen and clear some of the snot congesting him. You can also use a baby nebulizer or humidifer, but often cats don’t like things held up to their faces. If he doesn't, then you can make a little ‘steam tent’ with him in his carrier with a sheet over that and the humidifier.
If he is building up mucus that the steam isn't shifting, you can use a cotton ball moistened with warm water to wipe away crust and mucus. Furthermore, you can use saline nasal drops like Ocean Mist (but not anything medicated) to further reduce discharge build up. To do so, just tilt his head back and drop two to three drops in one nostril. Cats hate this, but it helps. After the drops go down, you can let the head up and wipe away any discharge that gets loosened. Then repeat with the other nostril.
Making sure he is getting food and water is important, as congested cats who can’t smell their food often won’t eat as well as they should. Since Monty is off his food, you do want to make sure to keep tempting with smelly wet foods to help break through that congestion and keep them eating. As well, it can help to warm it up a bit in the microwave to help him be able to smell it.
Just to note, if he has been off his food for a few days and tempting doesn't help, then you may need to consider initiating 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. It too is available OTC at the vets, some pet stores, and online. Furthermore, just as a short term option while securing these, you can consider offering kitten wet food (since there is more nutrition per bite of food) and if he isn't keen then this can be watered down to a gruel for syringe feeding as well.
Finally, if he has a history of herpes exposure or you just want to rule it out, you can consider treating him with L-lysine. This is an OTC nutritional supplement that can help them recover quicker. This is available over the counter at health food stores, pet stores, and the vets. They tend to come as huge tablets, so I advise crushing them and mixing it with food. An average cat dose is 500mg a day.
Overall, it sounds like poor wee lad has caught one of the cat flu agents. Therefore, I would advise starting the above supportive care measures and supplement his food intake if he won't. If you do this, but if the discharges he is producing are snotty or his signs are severe (and the above supportive care doesn't get his breathing more comfortably), then that will be your cue to visit the vet to get his checked out. The vet will be able to examine him, dispense antibiotics for any bacterial agents and provide cat safe decongestants (since humans ones are toxic to kitties) to get his back on track and settle this for him.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,