@Bethany92I had the SAME problems with a betta we bought for my son! His name is Cutie. The kids pick the best names. I came to the forum for help back in January and it was great. See my previous posts on it.
Recs from my experience - I found Kanaplex to work best. Since fin rot is largely caused by gram negative bacteria and Kanaplex covers more gram negative baceteria than Maracyn. (Gram positive and gram negative bacteria are classifications for different bacteria).
I also found that the kanaplex really started to work wonders when i got the salt up to 1 TBSP per gallon. I started at 1 TBSP per 5 gallon, then went to 1 TBSP per 3 gallons, then 1 TBSP per 1 gallon.
Check out this link from AC. https://www.aquariumcoop.com/blogs/aquarium/aquarium-salt-for-sick-fish
Finally, i increased the temp of his tank to 80F to reduce any stress he was feeling.
i think you still have a chance to save him!