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.
11 cockatiels, 1 green cheek conure, 1 dog, 1 hamster, 10 G fish tank, 5 G fish tank.