Is your tank cycled? Ammonia isn't THAT high. Do a 50% change anyways. But before that, can you test nitrites? If you didn't cycle the tank and you're going through your nitrite spike you'll need to do tons of WCs, nitrite is more toxic to fish than ammonia. You should also test nitrates.
Don't treat him with AQ salt, it's bad for cories.. Someone else may have some advice on what you can use.
This is my new 20 gallon long tank. I had a 10 gallon tank before fully cycled, so i used the water, gravel, filter from that tank and added an extra filter, all my tests are okay and i did a 50% water change yesterday. Could it have something to do with the fact that i added 2 platy yesterday?
I think you'd see physical damage, the stress would have worn off by now.
Just keep the water really clean, hopefully he will pull through. If you have a different tank, even like a bowl, if you have a heater, put him in, in case it's contagious. Or even float him in a plastic container in the main tank :/