Thanks for the advices guys, currently i transferred him to a bigger glass tank filled with dechlorinated tap water. He seems fine so far, better than on the previous water just swimming around the new bigger empty tank. I realize the previous water is bit cold and when i checked carefully the water got some soft barely visible tiny dust, i guess the rainwater is contaminated..My bad
Just now I adjusted the temperature on the new tank before acclimating him.
So far no more itchy problem, hope it stays that way. If it occur again soon, I'll do what Myates suggested.
Oldfishlady - I do the Float the cup method for acclimating. I just got a heater so i can now adjust the temperature. The only thing missing is the kit for checking and adjusting pH..Need to get that one once i got the cash.
NeptunesMom - I live in Borneo island, the home of Mouthbrooders Betta macrostoma. You got the point actually, my brother just remind me of the Liquefied Gas factory not far from here so we can expect a polluted rain/acid rain..
Thanks everyone, will update on my betta condition again.