Cat Health Questions? Ask a Cat Vet for Answers ASAP.
Believe me, I know from first hand experience how frustrating this can be. My own youngest cat, Beastly, has a problem with nasal discharge, and it often gets so bad he is sneezing blood all over the house. I feel like I have done everything on earth for him, except MRI and I can't get rid of the problem. But in the end, if your cat has a chronic sinus infection, then managing symptoms may be the best we can do.
Ok, so what can you do? There are a large number of tests out there that you can do if you want to verify exactly what is going on with Milo. You can culture the discharge to get an idea what is growing up there. You can take a swab and do a viral test to check which of the viral infections may be causing these signs. You can do advanced imaging such as CT or MRI to make sure there is no polyp, fungal mass, bony loss, etc that is complicating or causing these problems. But in the vast majority of cases of cats with nasal discharge, the end result is going to be a chronic sinusitis. Viral infections can be nasty, and they are hard to deal with, because we can't "cure" them. Antibiotics only work on bacterial infections. Viral infections require the immune system to knock them out (think of us with the flu, for example).
You may already have done some of this, but bear with me:
Step 1--Make sure Milo isn't immune compromised. This involves FeLV/FIV testing, etc. His immune system has to be 100% to help us fight this.
Step 2--Treat any bacterial infections, if present. This may require culturing the nose, topicals for the eye, if they are affected.
Step 3--Consider anti viral medications (although $$$). I have used Famcyclovir in the past in my cats, but there are others. Also, the supplement L-Lysine is safe and the powder form can be sprinkled on the food daily (sold over the counter).
Step 4--Nasal drops--cats HATE these, but when Beastly gets really bad, I will sometimes treat with either children's neo-synephrine (one drop in alternate nostrils every day), or when he gets really bad, I do sometimes use a preparation of ophthalmic antibiotic in saline solution that I drip into the nose (you would have to discuss doing this with your vet--it's not a common thing to do, I learned about it from a feline vet who has been in practice for 20+ years).
Step 5--Hope that these things will allow the symptoms to subside, and be prepared for any flareups when and if they occur. I hope this helps a bit, please let me know if I can answer any other questions.