Hi Mystus,
I have some info/questions that may help to further diagnose the problem. I'm not sure how familiar you are with the swim bladder issues, so I am including some info I have picked up on researching the issue. You may already know all of this.. if so, then hopefully others can benefit from it.
The swim bladder problem typically comes from the sphincter valve not being able to properly regulate the pressure inside the bladder. Either air/gas can't get in (fish on the bottom of the tank, typically on its side, or fish swimming sideways), or it can't get out efficiently (floaters, or upside down swimmers). So, the question is: Why is the valve not working correctly? If you can figure that out, you can be more directed in your treatment.
I have read that a bacterial infection, constipation, and internal parasites can all lead to swim bladder issues. Some of the more popular remedies are treating with Erythromycin for bacterial infection, epsom salt for constipation, and flubendazole for internal parasites (this is a dewormer). I have not read anywhere that fungus could be causing it, so you may wan't to discontinue the anti-fungal treatments unless you actually see fungus.
Constipation:
Now that he is in a quarantine tank, have you seen him poop? (a bare bottom should make it pretty easy to tell) If so, then you can rule out constipation. If not... then maybe that is the issue. Constipation can also lead to not eating... for obvious reasons. So, if the force feeding worked at all, and you don't see any waste, that may be the problem. Obviously, if he isn't eating, then he wouldn't be pooping... so, not pooping doesn't rule out the other issues.
For treatment, there are lots of ideas out there. I mentioned epsom salt earlier (which can be added to food, although I have never tried it), I have also heard about peas. Brine shrimp have also done the trick for some of my discus. You may want to look into an epsom salt bath too. I have read about it, but can't remember the details.
One of the other recommendations I have read is to lower the water level of the tank. In theory a shorter water column will exert less pressure on the fish, which may give it a rest for a while. This change in relative pressure may help move things along, much like your ears popping when on an airplane. My only concern with lowering the water level is that as there is less pressure, the trapped gas can expand. I would think that this doesn't feel all that great. At the same time... it wouldn't be any worse the the fish coming up the to the top of the tank on its own... so its probably just fine to try it.
If constipation isn't the issue, then it could be either bacterial, or parasites, or both. I'm not really sure if there is a good way to diagnose which it is.
Bacterial:
For bacterial infections, you could try Erythromycin as a bath. But this doesn't target the area specifically. Aquarium Products (AP) has a product line called gel.tek. Here is a link to their info:
http://www.aq-products.com/aquamed_GELTEK.htm
They have many of the same antibiotics such as Erythromycin that you can deliver directly to the fish via their food. The directions say to squirt drops into the tank and the fish will eat it... yeah right! But, you can also soak your food in it, and freeze it. If the fish isn't eating, you can load the stuff into a syringe (no needle, haha) and give them a gutshot. I have done this on even the smallest discus, and once you try it a few times, it becomes much less scary. Jen, my wife, is a veterinary technician, and she has even used a syringe (with a needle) to inject nightcrawlers with the stuff to feed to our big fire eel when he got sick. It worked very well. The key for the discus was to also have an additional tube on the end of the syringe to get the delivery further back into the gut. On something the size of a TSN, I would think you could use silicone airline tubing stuck to the end of a 25 or 35 cc syringe. Just make sure the tube is on there good. Once you get the tip of the tube past the gills, much more of the medicine will get into the stomach. By the way, you can also mash up thawed frozen blood worms into a paste, mix with the medicine, and inject the whole mix.
Parasites:
It could also be that there is a log jam of worms causing the problem. In this case, you need flubendazole. I think a derivative of this may be in Jungle Labs "parasite clear." Although as a bath, it is not as effective as a direct dose. The gel.tek line has a product called "Ultra Cure PX" that contains flubendazole, Metronidazole and Praziquantel. It basically takes care of any type of internal parasite. The delivery method is the same for all the gel.tek products, so you can mix it and freeze it, or just mix it, or inject it. I add it the discus food from time to time to keep them clean.
So, hopefully some of this has been useful. I haven't tried everything that I listed above; it is just the general ideas that I found consistently through my research. So, make sure you look into everything further for yourself before choosing a treatment option.
Here is a link to a page that discusses some of the treatment options for discus:
http://article.discusnews.com/cat-02/swimblader.shtml
Swim bladder is a fairly common issue with discus, and although the anatomy of TSN and discus are very different... they are still fish, and they come from South America with reasonably similar water conditions. So, you might find some of the info useful.
-Eric