developmental delays

My little guy just turned 10 months old. He will occasionally put pressure on his legs but not often and can't sit up unassisted yet and doesn't crawl. I should add that we our being followed by his doctor and just started therapy. He's also going for an MRI next week. 
He was born a month early and was healthy at birth but I did have placenta problems as well and I'm not sure if that has to do with his delays. 
I'm just wondering if anyone else has had a baby that has been a late sitter and eventually caught up? 
Picture of my happy guy below!