I can't tell from the picture - and I am by far no expert but I'm pretty sure that fin rot is mostly brought on from stress. How long have you had him? If you had him for a while and this is new, you may want to check your parameters, could be that something is off. Or it could be a long term effect of diet, if so vitamin supplements may help or some additional variety in the diet including some slightly steamed broccoli. BTW it's a beautiful looking fish.
I'm sure others will chime in that know more. Good luck