When I first got my betta I had to keep him in a .5 gallon tank for a bit.
He was much like yours - mostly inactive, but otherwise looked perfectly healthy. He ate normally and acknowledged my presence with some wiggling, but spent the majority of his time resting.
Once I got him into his 5 gallon tank, he goes crazy all day.
I think your fish just doesn't have room to move! He may also be cold and bored.
I suggest you read this thread http://www.bettafish.com/showthread.php?t=20058
Edit: Also, pleeaaase change the water at least every other day, 100%, while he's in that tank. I tested my water daily and found that every other day was the time when the quality just started to deteriorate. Make sure you're conditioning it and getting the same temperature.