Welcome to the Wonderful World of Tailbiting. Yes, it's frustrating. And sometimes, just when you think you've solved the issue, the little guy may decide it's time for a new 'haircut' again.
One of my guys does tailbite when he's hungry. He gets two meals per day, with no fasting days. I've found he also needs:
- excellent water quality
- floating plants or a betta hammock to sleep in
- complete darkness at night
- low dose Epsom salt for a day or two if he starts to become constipated. (If he doesn't poop, he rips off his fins.)
- a neighbor to watch. (He actually does better if he can see another betta. Maybe it just keeps him busy, so he doesn't have time to rip off his fins....)
If I keep him in those conditions, he tends not to rip off his fins. (But tomorrow is always another day....)
As for moving your guy into the larger tank.... This may work, and it may not. Some fish like having more room. But when I moved my guy into a larger tank, he wanted to swim more - which meant the fins got in the way. So he ripped them off. Again.
I really think universities need to start offering degrees in "Fish Psychology."