Hi breeglet. So sorry to hear about the problems your guy is having.
You said the white patches look more like indentations in the scales than thick patches ON TOP of the scales? Just curious but have you tried taking him, very briefly out of water? If they were indeed indentations, it might show up clearer out of water.
My initial reaction is it is either columnaris (flexibacter columnaris) or an external parasite such as tetrahyema, which often appears in conjunction with flexibacter. Both can cause ulcerations which may be what we're seeing here. It's possible that the tetrahyema caused enough significant damage to the scales and skin that flexibacter set in, causing the grayish patches over the scale loss.
Has treatment with the AQ salt done anything at all to help? I do prefer to try holistic methods first, especially when I'm not sure if I'm dealing with a parasite or a bacteria, but if the AQ salt isn't really doing anything then there are a few medical options, ranging in strength and harshness of treatment.
On the lowest end of the scale is daily baths in double strength methylene blue. 1 tsp per 5 gallons makes a double strength bath or I guess in this case, probably roughly 1/2 tsp in his 2 gal. A half hour a day, maybe a few minutes more if he shows no distress. Methylene blue can sometimes be quite effective in treating conditions that are hard to diagnose and is relatively harmless to the system.
You can also try a bath of potassium permanganate at roughly double the recommended dosage for 10 minutes a day (somewhere around 5-10ppm per gallon). Do not use a dechlorinator in the water if you use potassium permanganate, as the dechlorinator will just remove the PP. Carefully observe the fish for signs of distress. Do not use this in a planted tank; it will destroy your beneficial bacteria and it will also inhibit photosynthesis in your plants.
Next would be treating with API Fungus Cure. While columnaris isn't a true fungus, the combination of ingredients in Fungus Cure may prove effective in treating what could be both a parasite and a bacteria problem.
For the most heavy-duty punch, you would combine a parasitic product such as Seachem ParaGuard with a strong antibiotic such as Seachem Kanaplex. However, since it looks like your guy is still active and otherwise acting okay, I would not recommend this course just yet.