He was probably a little more active in the smaller tank because of the other male on the other side, not the size of the tank.
My guess is his lethargy before you started treatment was due to the cold water, bettas are tropical fish after all, but since that isn't the case now it could be a couple of things....
What is/was your regular water change schedule for the 1 gallon?
Thats some pretty heavy medicating for just some Ich....generally a couple of weeks in AQ salt at 83-85 should with daily water changes should do the trick.
How long were you treating for? When you moved him back into his 1 gallon, did you acclimate him back in? If so, how?
What sort of heater are you using? Does the temperature fluctuate? If so, how much? What sort of thermometer are you using? If you're heater is adjustable, go ahead and bump the temp down a couple of degrees over the next couple of hours to about 80. No need to keep it quite so high now that hes out of treatment.
The 'pink spots' from what I can see in the picture, look just like his normal coloring to me...though he does look a tad on the pale/sickly looking side.
Did you add conditioner into his water? What brand? Do you use regular tap water? Any chance any sort of soaps or chemicals could have gotten into his tank? Do you know your levels(Ammonia, Nitrite, Nitrate, PH)?