Betta Fish: Need help saving my daughter’s Betta, she’s had him 1 year, suddenly lethargic, I suspected fin rot?

My daughter has had her Betta fish for a year, she takes good care of him and I help with cleaning, changing water, etc. I noticed that suddenly he was not swimming as much, his fins looked really bad, he's not eating a lot, and he just wants to hide.

Attached photos are:
1. Day 1 when I saw he was sick,
2. Today, Day 4
3. Medication
4. Photo from a few months ago
5. Photo when she got him a year ago

I immediately tested the water with a 7 in 1 test and it was perfect. I thought based on his symptoms and lack of other obvious problems, it could be fin rot so I got medication and because his 10 gallon tank is planted and has a few snails, I moved him into a heated 2.5 gallon tank with new conditioned water, and we added a few soft fake decorations for hiding spots. I have followed the instructions on the medication, today is day 4 which is the last day of treatment. We also got a new food but it's dying the water red and he doesn't seem to be eating it either.

He will swim pretty fast still when I stick the net in, but otherwise he just lays on the bottom.

How long does it take for them to recover from fin rot? Could this be something else?