Hi Miish, I'm glad to hear he seems to be improving. I just want to mention that your tank looks like one of the Marina Zen tanks to me. http://www.amazon.com/Marina-Betta-P.../dp/B001B5CQ8W
If so, it isn't 2.5 gallons, it's only 0.5 of a gallon. If that's the case, more water changes are going to be necessary. Until you can get a larger tank, a Kritter Keeper or even a plastic storage bin (a couple of dollars at thrift shops for a 3 gallon) would make a great temporary home. :)
Do you have a hot tap? If so, run the new water for his tank warm. Don't bother letting it sit a few days. The Nutrafin stuff, if it is a water conditioner, will remove the chlorine, and letting it sit isn't doing anything.
When you have the funds, I strongly suggest getting hold of a really good conditioner. Prime is my favourite, but many members here also like StressCoat. :)