I'm very curious to know what breathing treatment the vet does and do you also do the same breathing treatments at home or something different?
Did the vet have any thoughts as to why he has gotten more than 1 RI?
Many animals (including humans) can be individually predisposed towards getting the same kind of infection over and over. If there is any immune deficit of any sort that could lead to repeated infections. Additionally once an infection has set in it is often easier for it to happen again in the future.
Just like in people; I'm sure there are folks reading this right now who get bronchitis every winter, have a constant psoriasis or eczema problem, often get athletes foot or other fungal infections over and over, etc.