You want to fill this out first, it'll make it easier for us to help you.
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
Temp; between 76 and 82. Ideal is 78 for most people. I stay around 78-80.
As for why he's being so still.. he could be lazy by personality, he could be sick. You need to check the tank's parameters and post those, and look at him closely for signs of illness. Check his scales, his fins, his mouth, his eyes and his gills. Fill out that form above and add anything you see.
Make sure he is pooping (easier to do without substrate) and make sure you aren't over feeding him. 2-3 pellets two times a day. If he's little, stick to 2. If you can't catch him pooping, fast him for a day. No food.
There is, of course, the possibility he's just getting used to the new tank, and needs some time. Don't move him back to the other tank, it's way way way too small, but make sure his new one has hiding places like caves and plants, so he feels more at home and safe. Silk or live plants work best.