FAT oscar, never poops!

Hi,
Ive got an oscar w/ a problem. He can't poop! Ive had him for a
couple of years now, and have never seen him poop. Occasonially he
cough's and throws up. I asked around and everyone at the local fish
stores thinks Im crazy.. they say he must be going to the bathroom,
and that I just dont see him do it. I keep a very close eye on my
aquarium, and would really know if he was. It was never much of a
problem, and I always had the hope in the back of my mind that I was
wrong, so never did anything about it. Now, he's starting to have
some problems. He has a HUGE buldge just before his vent. It looks
like there is an inch wide marble stuck in his gut. He's also just
kind of looking unhappy and sick lately.
I first noticed the problem because I'd see him hack and throw up
about once a week. I payed close attention to him and noticed that he
never went to the bathroom. At that time I tried all kinds of
medications and salt water dips, and just about everything my local
pet store would sell me, and nothing has made any difference. I
haven't seen him throw up in weeks now, and the buldge on his
underside just keeps getting bigger. Im afraid he may pop!
Does anyone have any ideas of what could be wrong? Could he have
something stuck in him blocking him up? if thats the case, is it
possible to have surgery on a fish??

Thanks