I don't personally recommend adding anything to the tank. Keep his water warm and clean and his fins will heal. Try placing interesting things outside his tank. Brightly colored objects work very well. Getting him to flare occasionally with a mirror is another way to combat boredom. Some bettas just bite which is why it is good to keep the water as clean as possible. Some hardwood leaves (like oak) or IAL are good for the tank as well in these situations.