Now with Slipper's having a rescue history and being a fairly young man, I'd be suspicious of a bacterial or viral upper airway infection. Allergies and polyps also could fit but nasal tumors would be less likely. And for the skin lesions, that does sound like an allergic reaction to flea saliva.
With this all in mind, we can start some supportive care. For the congestion signs, we can try steam treatment to start. To do so, you ca 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 they 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.
And if 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 as a gel, powder, or crushable tablets to add to food. Most health food stores, pet stores, and the vets carry this and the average cat dose is 500mg a day.
For the flea allergies, we'd want to use a good quality kitty specific flea treatment (ie Advantage Multi, Revolution, Capstar, etc) and to counter the allergy from it (why they itch), we can use an OTC antihistamine. Most commonly we use Benadryl/Diphenhydramine (0.25mg per pound of their body weight every 8-12h hours) can just be enough to reduce that allergic irritation. Alternatively, you can also use Cetirizine (just 5mg for a cat) once daily. For either, we like to keep the dose low, since it can cause drowsiness (just like people). And of course, these shouldn't be used if your wee one has any known health issues or is on any other medication without speaking to your vet first.
Overall, it sounds like poor wee lad has caught one of the cat flu agents and may be a carrier having been exposed before you adopted him. And it does sound like he is flea sensitive. Therefore, I would advise starting the above supportive care measures. But if either signs linger or we see snotty discharge then we'd want his local vet to see 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.
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