If it's severe fin rot, I'd suggest maracin 2. If it's just a torn fin melafix should help with the healing process. I personally don't use it (I prefer Stress Coat and super clean water method). If you do chose to use melfix however make sure to use only half the recommended dose and watch your betta's breathing very very carefully. Sadly I attempted to use this on one of my boys and he was allergic to it (and didn't make it) which is why I personally don't use it.
If his breathing is effected at all be prepared to transfer him over to clean (declorinized of course) water.
I havn't used the Jungle Fungus Eliminator, but from what I've heard it's pretty good too. Hope this helps!!