PP here. We moved from a otc type medication to a controlled narcotic pain killer for the dog after initial treatment didn't work. My dog is 8 and I felt like I was putting him through a lot though he is a super healthy, active boy outside the eye issues....I'm not sure how much a 16 yr old can take.
Sounds like you're doing everything possible for your pup - I hope the ophthalmologist has some good advise for you!