Is there any white fuzzy stuff near the missing scales?
also, since 1 gallon isn't very big - maybe the jerky movement is because he can't really swim too far? The 1 gallon I have is a triangle shape and there really is not much room for horizontal swimming. When I do use it, the betta usually just swims back and forth, back and forth...
is it like he is having seizures?
also when you get the flashlight, look for something that looks like gold dust on him. Its a parasite called velvet but you usually cant see it unless a light is shined on them