Welcome to the forum. (:
If you're treating him for fin rot, the usual treatment is 1 tsp of aquarium salt per gallon with 100% water changes everyday for 7-10 days. You can treat him in the cup to make water changes easier. To keep his water warm, float his cup in his tank water with the heater on. You shouldn't use AQ salt without 100% water changes or on a daily basis if you're not treating him for anything.
When his fin rot is gone, I think two 50% water changes each week with a 100% water change every other week is fine, if your tank isn't cycled (but, I'm not completely sure about this, you should check with others).
It's not needed to always add API Stresscoat with water conditioner, but it's completely fine to do so. I do because my male is a tailbiter and I like to keep his tail clean and infection-free. And when dosing water conditioner, it's best to dose it for the full volume of the tank, so in your case, 3.5 gallons. You don't have to worry about overdosing, because it would take an ungodly amount to harm your fish.
And yes, it's completely normal if he hasn't eaten. Just give him time, and eventually he should eat. There have been instances where bettas haven't eaten for 14+ days after coming to a new home, so be patient. (: