The ph is fine the ammonia is at a lethal level. It's just completely out of control. I would siphon your water right down to the gravel and so as large of a change as possible using same temp water and an hour long acclimation while you float and add a few tablespoons of new water to his cup every 10 minutes.
After that he may still need another water change.. I really think you need a test kit to cycle this safely or else abandon the idea until you can afford it and do twice weekly 50% 100% changes.
I wouldn't use the betta fix or aquarium salt at this point. Let's see how some clean water works.. it may just be ammonia burns. But if his fins continue to deteriorate he'll need treatment.