What are ways I can help my little guy sit up? NOT with a bumbo seat! I've read awful things about displaced hips,…

What are ways I can help my little guy sit up? NOT with a bumbo seat! I've read awful things about displaced hips, discomfort, and the seat tipping backwards and babies fracturing their skull because of it, so we won't be using one with Henry. What are some other things we can try? He was born at 37 weeks but because he stopped growing in the womb, so development wise he was on track with a 32 week old fetus Instead of 37 weeks, so he's a bit behind on milestones. Which I had planned for, but he shows strong interest in sitting up and can lift his back away from the back of his swing. But he can't balance himself yet. He also doesn't seem to want to completely straighten his legs when he sits? Anyone know what's up with that? TIA!