It is very common for puffers to go blind and I have had a few with that problem but they all recoperated. It is thought that too bright light causes this, I am not sure I believe that as they are coral reef fish and live in bright light. I would not give him medication, just meaty food like clams and worms if you can get them. Live, not freeze dried. I think if you give him clams for a week or two he will recover. Freeze a clam and scrape off thin slices, especially the dark parts as that is the clams gute where all the nutrition is. When you feed a shrimp from a grogery store you are just feeding muscle, there is very little nutrition in that, thats why I recommend clams as you are using the entire animal, guts and all. Thats what your fish needs as I think this comes about from a lack of the proper diet. Puffers have huge eyes and it takes a lot of nutrients to keep those eyes healthy.