Poor guy--is there any way you can lower the water level so that he only has a couple inches to go for a breath? This should help him immensely. Adding things for him to sit on close to the surface, such as an inverted coffee mug or a plastic disposable cup with some gravel on the top to weight it down will give him something to perch on near the surface.
If you have an air pump and airstone, it would be a good idea to aerate the water gently. Tie knots in the airline tubing until only a few bubbles come out at once. The current caused by a steady stream of bubbles would likely stress him out more.
Given the fact that the poor guy is not eating, in my mind there is only one real treatment option, and that is a dewormer. When fish have intestinal worms, they often lose their appetite very quickly and slowly deteriorate. If this were an internal bacterial infection, he would likely already be dead at this point, but if it is a bacterial infection, he is likely past the point of saving--it is quite rare for a fish to survive two weeks of infection without any real treatment. I think that all you can really do is use a dewormer with the ingredients praziquantel and metronidazole, such as Jungle's Parasite Clear dissolving tablets, and hope for the best. If it is internal parasites that he is afflicted with, he should start passing the worms within about 4-5 days, and once that has occurred, typically they start eating and rapidly recovering.