Treat for internal parasites. Jungle parasite clear or seachem metro, polyguard, or paraguard seem to work pretty well. You can also get medicated food. Just go look at your LFS, and read the lables to see what covers the largest range of IP's. Lots of companies make good IP meds. I am partial to seachem chemicals as I think they make good products.
Don't forget to follow all of the directions. You'll likely have to take out any carbon you may have in yoru filter and do lots of water changes along with the meds.
If that doesn't work, it may be internal bacteria, or something else. But from my experience, a lot of GT's are prone to IP's, so I figure it'll be your best bet to treat for IP's anyway since he's got sunken gut.
Does he have white or clear feces? That's a sure sign that there's some parasite issues going on.