Is he able to climb and perch on your finger? The reason sick birds sit on the floor is because they no longer can balance and are too weak to hold onto a perch. If he is not weak and his balance is fine and all other behaviors are normal then i'd bet you just have a strange bird.
You should hang some millet beside his perches to encourage him to perch. Or put a perch as low as you can get it in his cage. It could be that he was never taught to perch and he might need some encouragement from you.
9 cockatiels, 1 green cheek conure, 1 dog, 1 hamster, 10 G fish tank.