Unless you've already cycled
(that link is an information guide on what exactly an aquarium cycle is and how to cycle your tank. I strongly suggest you read it. It's got tons of great info in it) your tank or you are in the process of fish-in cycling I suggest you do the 100% changes or move him to a temporary 1 gallon container and do 100% changes in that.
Also a 2.5 gallon is way too small for a betta and a cory cat fish. Bettas get to be about 2-3 inches in length so they need 2.5 gallons of water to themselves.
Cory cat's get to be about 3 inches in size so they need at least 3 gallons of water to live in, combine that with the fact that they are shoaling (or schooling) fish and need to be kept in numbers of at least 3 or more, you really need 9 gallons just for the cory cats.
You could house a betta by himself in the 2.5 gallon but that's it. I suggest either taking the cory cat back or getting a 10 gallon, 2 more corries, cycling the 10 gallon, and keeping the 2.5 gallon as a hospital tank to treat your boy for finrot in.
The reason I say this is that if you have them in a smaller container then the ammonia that they secrete from their gills (which is highly toxic to them) will build up so rapidly that it will cause issues like finrot and enflamed gills. (unless you are doing 100% daily changes, even a cycled 2.5 gallon wouldn't be able to handle all that waste in such a small space(and your boy seems to already be sick with finrot)) You need to up your water changes.
But yes 1 tsp per gallon Aqua Salt and frequent 100% water changes should have your boy back in tip top shape in no time! =]
Good luck treating him and cycling your tank! =]