Your bettas' actions will probably be the best indicator of his health. If he seems to be acting normal, I would go with mild/homeopathic options...it may even just be coloration - stressed fish in a cup is normally going to undergo at least a bit of a color change when put into a good living space. I bought a "cambodian" colored young veiltail male and *literally* within one day of being put into his tank, he was bright solid red.
Gentle remedies I've had good luck with for external issues:
If his behaviour changes, that's usually a good sign something is going on with the fish.
FWIW: My daughter's crowntail got bonked with the water change cup some months back - I put him in AQ salt to help heal it up and he actually acted sluggish and meh for the first two days of that, so be aware a given treatment itself might produce some change in level of activity.
Good luck - I hope it's nothing, but it's good you are looking into it
He's a pretty fish!