Persian Cat Ear Issue?

Hi, I rescued a Persian Cat 6 months ago. He has this brown crusty stuff and goop in one ear. We have had him to the vet at least 12 times and have even done a surgery to clean it out. The only time it gets better is when we put medication in it, but as soon as we stop the medication, the crust and goo come back. It is not earmites and we tried changing his food, but again as soon as we stopped the drops, it came back. One would think that if we knew what is curing it, that we could diagnose it, but.....At this point I am cleaning it out daily with Epi-Otic and went back to using some medicine. Has anyone ever had this problem before?? If so, what have you done? Again, he has been to several vets and has been on multiple medications, both oral and topical. Please help.