That's why it's better to get a larger tank with some decorations, once every couple weeks switch them out to give them new places to explore. Just make sure your decorations don't have sharp edges where their fins can get torn/caught.
When I almost cooked my bettas by leaving them out by the windowsill, I noticed my male halfmoon was acting strange. Instead of swimming around, he was hiding.
When I found him, he was clinging to the side of the log decoration. Thought it was weird, so I put a thermometer in the tank he was in and the water was almost 100 degrees.
I'm lucky I moved his tank out with the new bettas, otherwise half of the new additions would been cooked by the time I got home in the evening.
Just sounds like your betta is bored right now.
Once your larger tank is cycled, he should liven up.