7 months barely sitting

My son was born at 35 weeks and he is now 7 months as of this past Saturday (6 months adjusted age) and some days he’ll sit for a few seconds to a minute or 2, but when I put him in a sitting position he scoots his butt forward and throws himself back. When I put a hoppy around him he just throws himself back to lounge and not sit, same thing when I sit him on the floor between my legs and he just scoots his butt forward and throws himself back. He is 20lbs and we did just start consistent tummy time daily 1-1.5 hours a day every day about 3-4 weeks ago. Before that he was getting around 15-30 mins of tummy time a day. Anybody have any suggestions on helping him to sit assisted and unassisted? Feeling defeated