I remember seeing Big Boy years ago on another forum, and it was a nice surprise to see him pop up here on MFK so much later.
I'm not squeamish about euthanising fish and other animals which seem to be suffering and have no hope of recovery...but I'm not entirely sure that is the case here. Twelve years is an impressive lifespan for a tank-raised Oscar, absolutely, but I think that's mostly because the vast majority of keepers kill their fish off prematurely. I wonder if it is really near the maximum lifespan that this fish could be expected to live. You've obviously taken excellent care of him...certainly can't see any of the all-too-common HITH disease, at least not in that pic.
I'm the worst person to ask for advice regarding fish medication, but we have lots of folks here who are well-versed in the use of medicines. It seems that I have read a lot about using epsom salts to treat some types of swim bladder malfunctions, and I'm sure that there are other things worth trying as well. I think it's too early to give up on the big guy.
kno4te
RD.
wednesday13
D
dogofwar
thebiggerthebetter
are a few that leap to mind who might be of assistance, or who might point the way towards others. Good luck!