Sounds like you are doing all you can for him at the moment, just keep water sparkly clean and maybe add a dash of aquarium salt as per instructions on the container. Any betta can get fin rot if water conditions are not optimum I guess
Awww poor thing. No, none of my CT's have ever had finrot. One of my PK girls has it currently, more due to her weakness since the big infection of April. I use Interpets Fungus and finrot med and have found it very good. Clean water is very important obviously so you're doing the right thing. Hopefully he will be ok.
P.