I just found the 'sick fish questions'. Here is more info:
Approx. 1.5 liter bowl
Temp: unknown, it's in an office
No filter, no other aeration, no heat
As noted before, no mates except a plant
Hikari Betta Bio-Gold
Amount? This is where it is tricky. I feed him every couple of days, 2-3 grains. However, because I was on vacation, I suspect he may have gotten more food despite my explicit instructions not to overfeed him.
Water changes usually once a week
100% water change
Additive: Aqueon Betta Bowl Plus -- a few drops with each change
Water has not been tested.
most of this is noted above. No history of any illness. Since I posted my question, I have moved him out of his bowl to a "hospital intensive care unit". It is a small square container, less water and I put warmer water in it.
His swimming is definitely lopsided, and it seems he is having trouble holding his weight...back end sinks fast and he struggles to stay vertical. However, he does seem to be coming to the surface more now in the smaller container.
The poor guy is really sick.