Strange Betta behaviour

This spazzy behavior is not uncommon in bettas and usually indicates a
physical problem with the environment and not a mental issue. It can be
caused by ammonia, nitrite or nitrate poisoning, exposure to chlorine
chloramines or heavy metals or even parasites. It's really hard to
guess without more information on your tank and fish care.

Can you tell us more about his living arrangments.

Tank Size
Is the tank cycled?
Amount of water you change and how often ?
What water conditioners are you using?
What kind, how often and how much food are you feeding him?
What is the temperature and are you using a heater to keep it stable?
Have you noticed anything physical on his body, white spots, soars or
cottonlike tufts?
Most importantly, do you test your tank/bowl water for ammonia, ph,
nitrite, nitrate, gh, and kh? If yes, what are the results? If not,
please consider buying test kits right away.

Every fish keeper must test the water regularly even if they only keep
one fish in a bowl at home. Bettas are very sensitive to changes in
their water and for some reason fish store employees hardly ever tell
their customers that they must be testing weekly and at every water
change.

I've heard of this behavior many times before. Go ahead and answer the
above questions as best as you can and I suspect we'll be able to at
least find out what is causing the problem. (one step closer to fixing
it)