Frequent water changes is not always a god thing, no, as sterile water allows the fish no time to build resistance to certain bacteria. However, your using a tank that is much too small for the one fish to live long term, thereby, the ammonia alone requires you to change water every two days, not every day because the water is too clean and stressful.
The IAL is something you can use for a month or so (each leaf lasts a long time). The longer it sits in the water the more helpful it is. SO, since you are doing such frequent changes you want to set up a bowl of IAL tinted water. To do this, get a bowl or bucket or something, condition it normally and toss in 3-4 leaves. Within a few hours it will be tanned pretty heavily. Use this tanned water as your water for when you change it. The tannins built up in it will help him get better. The darker the water, the better.
aq. salt can be used, however, it might not be necessary at this point. Since bettas are freshwater fish the aq salt should be preserved and used for more immediate concerns. If you want to use it because you are worried about his red tail, only mix 1tsp of aquarium salt to each gallon (16 US cups) of water and mix well. To make it easier for yourself, you can add the salt to the IAL water. This should be only used for 10 days.
(*IAL can and should be used at all times, aq salt should only be used for a max of 10 days)