7 month old--Not Sitting, Curved Spine?

M

My son is 7 months is week. Still not sitting up unsupported. Can go probably 3 seconds before he starts leaning.

Question. His spine literally looks so curve when he sits. I try to sit him up more but he just keeps going back to this position.

I noticed a slight bulge on his back since he was born. Every doctors appointment, I'd ask the doctor to check and she always say nothing is wrong with it.

Part of me is getting worried. He had slight torticollis when he was way younger and I am wondering if that is playing a part of him not sitting up too. We end up not having to do therapy at all because according to his doctor, he's all good and does good holding his head up.

We have been doing a lot of floor time and I have been sitting him up multiple times a day.

Any advice mommies?