You said that he has trouble swimming and keeping himself upright.... Is his abdomen area bloated at all?
If he appears bloated and has buoyancy issues, I would:
a) Stop the aquarium salt. (It can cause these problems.)
b) Switch to Epsom salt. (It has a fluid reducing and laxative effect.) Start at a dosage of 0.5 teaspoons Epsom salt per gallon. If that doesn't help within 1-2 days, increase to 1 teaspoon per gallon.
c) Look for a medication that can treat internal infections. (For bacterial infections, I would use Kanaplex (kanamycin sulfate).... Or you could try metronidazole, which would help with anaerobic bacteria.)
I don't know where you live, but if you can find mosquito larvae, this would be a good food for him. It's easily digestible and contains the nutrients they need.... If you can get these, you can also look into getting frozen foods.