I once put my boy into water that was too cold for him yet. I thought he was going to die. I got some warmer water, added conditioner, and put him under a lamp (this was before I knew about heaters and y'know, the bright light stressing them out).
It happens, if he rebounds, then just be more careful with the temperature of the water when you change it in the future.
Warm fishy wishes!
PS, the boy this happened too is now over a year old, and still grumpying along.