I have a beta fish, and when I first got him, he wouldn't eat. He actually wouldn't eat for about 4 days! He just needs time to adjust!
My mom also has a beta and she bought him a special heater. It keeps the water at just the right temperature and he's as happy as ever

He even rests on top of it sometimes!
Also, a 1.5G is about the right size for a beta.. You can just go to WalMart or ACMoore and get a big glass jar.