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.
My roommate says I'll die by giant turtle.