i think something is or was ripping his fins and has turned into fin rot..keep watching his fins..here is a little more info and the link for where i got the info from with pictures.if it continues then i would say fin rot...walmart has very little selection of medication.luckily i live in walking distance of petsmart,petco and walmart.petsmart and walmart are in the same shopping center and petco is just about a mile or less down from that..i also have a neighborhood fish store.not even a half mile from me.
really the best for fin rot is what is in my previous reply.if it does continue to get worse.i would contact OFL(OldFishLady)shes great with these kinda things..if u can,tomorrow post another pic especially if it has gotten worse.1 of my girls has black around her fins like pen stripping.another 1 has blue.but the shape of ur guys fins doesnt seem like a natural color.i could be wrong though.
Betta’s fins and/or tail seem to be getting shorter and shorter. Or they seem to be falling apart and dissolving. There may be a darker color (or a reddish one) to the edge of the betta’s fins/tail. He may be still active and eating normally, or may have stopped eating, fins may be clumped, color may be pale."