The best thing to do for your betta is to give him a balanced diet with different types of food. Pellets, bloodworm, brine shrimp, etc. He'll be a very happy and happy fish if you provide him variety. To keep him active, show him a mirror for no more than 5 minutes every other day so he "fights" with his reflection. This will motivate him and give him a better self-esteem every time he "defeats" his rival. Also, give him an interesting tank with decoration, plants, places to hide. Bettas are curious fish, so, give him things to investigate. If you can't put decorations inside his tank, put them in the outside. Put a toy, a picture, let him see the TV from afar. Things with movement will catch his attention and keep him active. Just, be patient for a while. He's new, let him climate to his new tank, environment, surroundings and owners. Once he's settled in, treat him as a pet, not some kind of decoration, and you'll see what a special little friend you have.