hrm. I keep going back and fourth on what I think this is... a lot of his symptoms are kinda conflicting.

here's what I would do ~ move him back to the 1/2 gallon and keep it filled halfway with clean, dechlorinated, tested water. Add a clean, live java fern from a trusted source to help with cleaning the water. To me, a filter in a 1/2 gal is overkill. Filters shouldn't be in a Betta tank any less than 2-3g, because in a smaller tank the currect would throw them around. 1/2 gals need to be changed at least 2 times a week, filter or not, anyway. Keep him in a dark place with a very constant temp. Did you say he's in 1 cup of water? if so... waaay too little, that would stress him more, and its just about impossible to treat 1 cup of water. To me, it sounds like swim bladder problems ~ gasping and swollen abdomen. I dunno, though. Is he having any trouble swimming to the surface? Does he swim sideways or sink/float a lot?