Orion has made huge progress physically. He is sitting up with less and less support. When he is not trying to lay down in his favorite position, he accepts people holding him at his hips for sitting on the floor. And only last night, I took my hands away from his hips and he stay seated for 1, maybe 2 seconds before toppling over sideways!
I learned that children like Orion feel more secure with their body in contact with something. There is contact from head to feet while laying on the floor. When sitting, there is less contact, with his bottom, legs, feet and maybe hands if he's propping himself up with them.