Often when a Betta has a change in behavior after a water change it can be due to several things......
If 100% and the fish removed-improper acclimation to the new chemistry-osmotic shock
If the tank was neglected and then large water change made-pH, nitrate shock could result
Not enough or forgotten dechlorinator used-chemical burns/shock
Too hot or too cold replacement water-temp shock
Heater not plugged in-too cold
Check your water temp to ensure he has the proper water temp and make a 50% water change dosing the dechlorinator for the full volume of the tank...be sure the replacement water is within a few degrees between new and old water....
Good luck and keep us posted.....
