Sorry I didn't see your post right away. Do his fins look ragged and disintegrating, or like they have large chunks taken out of them? Some bettas bite their own tails for some reason or another. Even if it appears to be fin rot now, he may have bitten his own tail which then got infected. If your plants are silk, they really shouldn't be a problem. Do you have a filter in there that he may have had a run in with? Of course, not being there I could be way off base, so the first priority now is to get the fin rot healing and then identify the cause. What medication are you using? A lot of pet stores recommend bettafix or melafix, beside my own bad experiences with it, you can read about it here: FishGeeks :: View topic - What you need to know about Bettafix and Melafix
I have had good luck with Fungus Clear by Jungle Labs, but you will find that everyone has their own opinions on the matter. This is just what has worked for me. Good luck.