hrm perhaps he could have cut himself at some point? I had a betta with that problem before, I kept thinking it was fin rot but it was the plant.
If you can find silk plants they are great!
It helped my situation at least. Your plant may be okay if its from the betta starter kits I know which plant you mean I had one way back...those plants seem okay I would say, unless you find yours feels kinda sharp. I think if you have a walmart that sells silk plants for cheap you could replace the plastic one if you feel yours is too sharp. My experience with those ones though is they seemed fine.
The rock should be okay now that the sharp plants are gone
keep us updated on your progress after the salt baths
hopefully they can help