Is he lethargic? Heavy breathing, floating at the top of laying on the bottom of the tank??
do you have any idea of the water temp? some of my guys were a bit clamped till i moved them into a divided and heated 10 gallon tank.
Im also thinking he may be stressed being in that tank. They like places to hide like caves and plants to swim around and perch on.
He is in one of these things right?
can you move him into something that is at least a gallon? It doesnt really matter what, as long as he can't jump out of it (saran wrap with holes can be used as a cover) and add a place for him to hide ( a coffee cup will work). Maybe that will perk him up a bit?
In general, bettas seem to like having tanks with lots of "stuff" in their tanks as opposed to one that is bare.