How are his blood tests and everything? What exactly did your vets prescribe? What reason, if any, did they give for this happening to him?
He's pretty old. It may be that his immune system is compromised and the infection won't go away or keeps recurring. Drops and ointment aren't going to help if it's an immune issue or if the infection has strongly taken hold. I'm surprised that your vets don't take this seriously and haven't prescribed any oral medication such as an antibiotic. The eye issues have to be very uncomfortable for him.
It also could be an allergic reaction to something, such as an ingredient in his food. Even if he has been on something for a long time he can develop an allergy. It's not uncommon for an older dog to develop an allergy to an ingredient after consuming it for a long time. What kind of food is he eating?