looks like sbd but heavy breathing and darting confusing to me. Darting and heavy breathing can be due to the gill flukes. You can still keep him in 2 gall tank and do the treatment . Fast him and see if he is better but is he will continue to dart and looks like he is gasping for air i would think he has flukes .
I would have Epsom salt and Aquarium salt on hands in case you need it. Is there any way you can maintain and raise the temperature later if you need to in 2 gall tank? If he will still have heavy breathing i would try to raise to raise the temp gradually about 1-2* an hour to 84* which speed up parasites life cycle and help them to fall faster. And in this situation you need to use aq salt though.
And i don't know how everyone else think but i would try to do 50% water changes now and see if it helps. If you will do water changes make sure new water approximately same temperature as his water.
Just read your thread again, sorry i missed that you are you using aquarium salt? How long and how much?