I would make a 100% water change and take out the melafix..it might be what's causing him to act so oddly and not eat.
When you fill up the tank, take some of the tank water and dissolve the salt in it, then add it back to the tank and gently stir it in.
Try not to use a net while he has fin rot..it can make it worse, trust me. Try to lure him to the surface and scoop him into a cup.
When you have the tank ready to go, float him in his betta cup in his tank. Add some of the tank water to his cup and siphon some of the water out, then wait 15 minutes. Then, add a little more water to his cup from the tank and siphon some of the water out and wait another 15 minutes. You can do this one more time, or if you don't want to, geeeently tilt his cup into the water and allow him to swim out on his own. This is one way to acclimate your fish to the new, fresh water and salt.
Yes, a rounded teaspoon is like a "heaping teaspoon", which means it goes over the top...which can equal 1.5 teaspoons instead of your desired 1 teaspoon. Good thing you level it off.
As for food, I would definitely go to the store and get some frozen bloodworms. Thaw them out at room temperature and using some tweezers or forceps, take some of the worms out and hold them in front of him in the water. The thawed frozen food will look like it's wiggling to him, which may spark his appetite.