I don't suppose you have photos? Spinal curve and sunken body are both symptoms of TB, and I have to wonder if the funny looking spots aren't the start of ulcers. For now, he should be ok on Quick Cure and Melafix, but if you can get pics that'd be great.
As for a 100% WC, I suggest leaving 3 gallons of water out overnight to reduce chlorine and get them to room temp. Then, remove the fish by gently scooping him into a clean container with a lid. Empty out his tank and, if it isn't cycled, clean the gravel to get out any feces and excess waste. Re-fill, and add dechlor and any other additives you regularly use. Then float your betta in the container like you would if you were acclimating a new fish, though it doesn't have to take quite as long. This will let him get used to the water temp, and any changes in water chemistry, without stressing him out too badly. Good luck, and I hope he feels better!
edit: Just saw that the tank is heated, so instead of letting the water sit to room temp overnight, you should instead try to get it out of tap at as close to the previous temp as possible. I'd fill it to 2g, and see how the temp is, then add cooler or warmer water if the difference from the prev. temp is more than 2 degrees F. Just make sure your dechlorinator works quickly; some - like amquel - can take a good 24 hours. Aquasafe supposedly dechlors "immediately," though I always give it at least 10 min.
