I did something similar by accident once. I ran warmer water (but not too warm) added a few drops of dechlorinator and put him in that. Also put him under a bright lamp until he warmed and perked up enough that he didn't look dead. I felt so bad.
Don't try to force him to move (even though we all have that urge). And just checking, did you remember to use decholorinator?
I'm sorry I'm not more help, ideally bettas should not have their water temperature waiver by more than about 2 degrees or so a day.
IF you could fill out this information we might be able to suggest more you could do for your little guy. Hope he pulls through, but shock like this can be just as hard on a betta as it is on us humans, or worse.
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?