Hi Rayner and welcome to the forum. Thursday is a really cute name. :)
Will he eat? Does he clamp his fins? From the way he is swimming, it could be that he is still recovering from the cold temps. When the temperature is as cold as 22 C, a betta expends all of his energy just staying alive so he may need some resting time. Ideally, 25-26 C would be best for him, if you can get it to that. I've used the same type of heater and it should get it there eventually.
If he's eating but he's not bloated, then go ahead and feed him as you normally do. This is a little gross, but monitor his poo to make sure he's not constipated and that it's a normal color. Whiteish poo can mean internal parasites or an internal bacterial infection.
Just to clarify, you don't want to add actual sea salt to the tank. By that, I mean you don't want to use the synthetic salts that are used to make a saltwater tank, such as Instant Ocean. Rather, you want to use aquarium salt that is specially made for medical purposes. The API brand has aquarium salt that should be readily available in most countries. You can also use any non-iodized table salt.
In this case however, you may want to add 1 tsp of epsom salts per gallon to his tank. Epsom salts are better for internal problems while aquarium salt is best for external problems like ich parasites and open wounds.
Hope this helps and I hope Thursday perks up as his water heats. Good luck!