It's okay, we all panic about our betta babies.
As long as the fin rot has not advanced to his body, your odds of curing it are close to 100%. Also, he probably isn't suffering if it's just in his fins. Do you see blackened edges on his fins and are his fins coming off in large chunks? If not, he may be tearing his fins on some sharp decor or plants or tailbiting.
The dosage of aquarium salt is 1 tsp per gallon, pre-dissolve in tank water before adding. A high-protein diet with lots of live or frozen foods is great, warm temps around 80-82*, and clean water will also help the fins heal fast.