I had 2 dwarf gouramis, then one died. like yours, he got very thin. and he appeared to shrink, as the other one grew and grew. he had no disease symptoms, other than he started to stop eating, and took to hiding and taking cover for long amounts of the day. when he got really bad,he becam lethargic, sitting in a plant, not moving until the bigger gourami chased him. after about 4 weeks, he got really bad. he didn't move for days on end, and began floating at the surface for hours. several times, i thought he was dead, but he wasn't. then 2 days later, his eyes went "rusty" and a ugly brown film clouded them over, but the other gourami pecked this away. he died later in the evening.
as gouramis are comercially bred, and the places where they are bred use tremendous amounts of antibiotics, there immune systems shut down, and i have heard they are succeptible to similar problems, even when the water is top quality, like mine.