im not sure what is causing it. here are some things that will help it heal..
kordons fish protector, way better then most slime coat replacers.
soak foods in garlic, galic helps to boost immune system. fresh garlic or seachem garlic guard
vita chem, adds vitamins can go directly into water or soak foods
extra water changes are always good.
what temp are you keeping him at? fin rot seems to slow down in lower temps. so maybe keep his temp around 78f if its not already.
good luck i hope this stops he sure is a pretty boy.
8g Fluval 1M CT Betta
125g 5 Blood parrots, 1 Green Severum, 8 Dension Barbs, 10 Yoyo Loaches, 1 RTS
75g Angelfish, 3 EBGR, 4 Siames Alage Eaters, 12 Harlequin Rasboars, 1 BN Pleco, 1 Swordtail
33g long cherry shrimp, 8 pygmy corys, 5 ottos
15g 2 GBR