Thanks so much for your helpful responses.
Darkmoon: Question about the epsom salts. Is he ok to be in them for 24 hours until I do my next water change?
Sakura: No offense taken at all ! thank you for your help. I was finally able to get a picture of his popeye, though it is kind of far away. But you can still see one eye is larger than the other.
I am worried about his fin rot and potential pop-eye, but the most perplexing thing to me of all is the tumor on his tail (you can kind of see it in the picture - the huge shiny blue lump at the end of his belly. Although he isn't keeping his tail clamped, but I'm wondering if maybe that's why he is hanging out at the bottom of his bowl so much. Because it's too heavy to swim with. Then darting directly up for air and back down again.
Thanks for all your help. I don't want to prolong his life if it is not the humane/comfortable thing to do. But if it is something that can be fixed...well i'm gonna try.