It does take time for fish to adjust to a different tank size. Have you thought of putting a small mirror next to the tank for the betta to look at? You can put it near a section of the tank where the flow is weaker as time goes by, you can position it closer to the stronger areas of flow. That way he can get used to it. A lot of people think using a mirror stressful, but I see it more like exercise time.
Edit- Don't leave the mirror up for hours at a time.
I already took him out. I felt bad for him and the stems couldn't get a chance to root because he kept knocking them loose. He's in a 6 gallon tank right now that's just a plant holding tank while i figure out what I want to do with him. I have three microdevario kubotai that I'll probably end up putting in here instead.
And yeah, I've read about using a mirror for exercise before, especially with showy finnage types like halfmoons. I've never bothered with him since he's a plakat and has room to swim.
Here's what the tank looks like now. I trimmed the stems some before I replanted them and the hydrocotyle is already growing quite a bit.