I’ve noticed that my 11.5 month old son leans back when he is sitting. Almost like he is sitting on his tailbone. Because of this, he is not as steady as he should be. He’ll often topple toward the side or backwards, but he’ll usually catch himself. He has been sitting since around 5 months and crawling since 6 months. He also pulls up to stand and has taken 2 steps. He was evaluated virtually by an occupational therapist and everything looked fine from her end, which is great. But what could be the reason for him sitting on his tailbone? Could it be a weak muscle issue if he is crawling and pulling to stand so proficiently? I’m confused.
Based on what you’re describing, I would try seating him on a boppy pillow, step stool, or an air cushion to see if he can manage that. Sitting on a smaller surface area encourages us to sit more upright. I’ll try to find a picture to add here
I would also try to have him side sit as that encourages him to turn on his core muscles more. You can also have him play at vertical surfaces so that he is reaching up which also helps with the posture you are describing!