My older cat, guessing around 15 keeps getting severely congested sinuses. He is constantly sneezing out mucus,hacking, and has very loud nasal breathing. I have taken him to 2 different vets and they say he isnt FIV+ and that he doesnt have anything majorly wrong. They just send him home with antibiotics which work for a while but then he gets sick again as soon he stops taking them. I started him on lysine powder the last timwe he got sick but it didnt seem to help because now he is sick again. Does anyone have any suggestions on supplements or anything that might make him better? Thank you all in advance!!