My ten month old son will not sit up on his own. He cowers his head oftentimes, but he can lift it up but then toppled over. He was a preemie at 36 and 4 and then at 5 weeks was failure to thrive…he is doing well health wise but I’m concerned about this specific development delay. He gets into crawl position and backs up across the room, he can stand in his jumper, but no sitting. Great at tummy time. I’ve been working with him each day helping him balance with my hands on his hips while sitting and we are using the bumbo but nothing is working well enough. Help😢