3 years is no comparison to 5 months, but there was a time period where I felt quite hopeless in treating Wick’s upper respiratory infection. What usually only took one to two courses of antibiotics for other rabbits took Wick 5 months. He’s a runt, and historically has shown his body is a bit weaker than others.
Have medical assessments not related to a URI been done? 13 years is a wonderful life and due to age, he could have an immune system that’s weakened, or there may be something else going on, making him use his resources elsewhere than fighting off the infection. Have you had blood work done?
I mention this because we had a user whose rabbit was taken over by a skin issue and it would not be cured. Only after the rabbit passed did the owner realize the rabbit had a tumor growing on the liver, constantly straining the systems.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.