It could be something more than a sinus infection if you've got a lump going there but I agree that if he's happy and eating, he's choosing life. That's his vote! An abscess like we're familiar with would actually be hot to the touch and it would be sore and painful to him, and it would break at some point.

When I was in a similar situation years ago (before I'd ever heard any cautions about using herbal medicine in cats), I had placed 2 fosters (3 month old kittens) with an agency that was supposed to take them to a big adoption fair in San Francisco the next day. Since San Francisco seems to never have any kittens available due to spay/neuter success there, and these kittens were gorgeous, it was expected they'd be placed. When I called after the weekend to hear good news, they told me that both kittens had gotten sick immediately and so were still there and hadn't been taken to the event. I drove out there and found both kittens with a whooping cough (both had been in perfect health before this); this was diagnosed at the vet as bortadella (kennel cough), and he told me that this particular shelter was plagued with it in both cats and dogs. The kittens had not been isolated either but put in with the general population, half of which were coughing like mad. The vet charged almost $200 for an antibiotic that didn't work, and then when I went back, charged the full amount all over again for both, to get a different antibiotic, that didn't work either. He said the type of kennel cough there was highly resistant as it had been so long established and treated so many times with so many drugs. So I was stuck. Soon my other cats were also coughing and had cold symptoms too and everyone was quite miserable. (And because of this period of ill health I was never able to place those kittens, they lived with me their whole lives.)

I worked all the time then but housing costs are so extreme here I couldn't take even myself to a doctor, much less afford another vet bill like this one. I'd started researching herbs so I could cure myself when I needed help, not being able to afford medical care. As I recall I gave all of the cats the same tincture I gave myself, which at the time consisted of echinacea and golden seal and other typical immune enhancers - and my only concern was that I needed to get the alcohol out of it by heating, etc., and dilute it a lot also, because the alcohol burns them and makes them foam at the mouth, they can't stand it. The tincture had good results within a few days, with lessening of all the symptoms to varying degrees among the cats. Some got better sooner than others, but the kittens, surprisingly, took the longest to get completely well - it took about 2 months. But it worked.

I also used herbs (this time I was more knowledgeable and researched those that could be safely used in cats) to keep my hyperthyroid cat going until such time as I could afford the expensive surgery he needed, after the vet's repeated blood tests had completely cleaned me out. There are lots of safe herbs to use in cats for this condition and they are effective. I ground up the herbs and put them in tiny #3 capsules for him and just guessed at the dosages as best I could. He was symptomatic so it was pretty easy to see when he was at a good level, neither rocketing around the place and losing weight, nor sleeping like the dead all the time. It took me a year to save up enough $.

I don't know the specifics on the use of echnicea, goldenseal, red root, yerba mansa, etc. in cats - but these are ingredients in the immune tincture I still use for myself. You could research this. I also use grapefruit seed extract (pill form ONLY, do not use the liquid on a cat ever) but I think this would be possibly toxic in a cat. It's extremely effective in humans for just about anything.

Whenever you google this, you get best results putting "name of herb, cats, feline" as your search terms. Without the commas.

If only it were possible to get saline solution up his nostrils but we can assume this is completely impossible!

Good luck with this - and just for general info., I keep records of dosage info on any standard antibiotics my cats take as many of these are available thru pet supply houses and you have to do some fancy calculations, but you can often save a vet bill when you know what's needed already. The only drawback is that a lot of things in cats have become somewhat or totally resistant to the older antibiotics. You should avoid flagyl, it can be quickly fatal even with a vet in charge. Again, it's always much better when you can feel that you're getting the professional advice and skill, but sometimes you're stuck without it and you need to live as you would if you were simply "off the grid". Whatever you try with your cat, smallest possible dose first, careful observation, good record keeping so you can go back and really recall exactly what did what and when.