I know I may get flamed for saying this, but maybe it is time. It really sounds like the fish is suffering and from what you tell us here, things are looking very bleak. I know that euthanising is hard, I had to euthanise one of mine about a year ago, he was pretty much seemed to be paralyzed at the bottom of the tank (then in the shallow bowl) and wouldn't move unless touched and even then it was him on his side, not eating etc. I tried all sorts of meds and nothing took effect.
I guess the way to look at it maybe, (again my opinion) is say a dog or cat was displaying the same. They would be euthanised. It is hard, and it hurts, but I suppose in the long run you are at least ending the pain and suffering. I know that some may think, but one day tomorrow he could be up and swimming again, believe me, I have thought the same, but I really don't think by the sounds of it that it will be the case.
The decision has to be yours though, don't let anyone talk you out of how you feel, whether to keep him alive or euthanise. The stuff above is only my opinion. It really sounds like you have tried everything to help him and I am sure he appreciates it.
I hope for the best for you