Yes, it definitely could just be that Niel, thats what I was thinking...but if I recall there was also some kind of external parasite that looks like long strings hanging from your fish. Just thought I'd mention it, just in case, you know?
Hmm...I'd suggest keeping him in that 2.5 for more then just a day, even if he is starting to look better. Its a lot easier to keep an eye on him and keep his water clean in the smaller tank; I'd say wait a few days at least, a week to be safe, of stable, normal behavior(activeness, willing to eat, greeting you when you come to see him, no clamping, a bit more vivid color, maybe getting him to flare at his reflection a couple times)and no visible signs of illness before trying the 10 gallon once again.
Also, maybe look into cycling your 10 gallon without fish? You could easily keep him in the 2.5 while cycling, and once the 10 gallon is stable and cycled you could add him back in once again.
^A nice little page that outlines the cycling process quite well.
Also, something I just thought of that I can't recall if I mentioned....what kind of hiding places/plants does he have in the 10? Bettas come from a dark, densely planted environment in the wild....without enough plants/caves they feel 'exposed' and therefore stressed....especially in a tank larger then 5 gallons, you'll want a lot of plants and caves to make him feel secure.