Hello. My Son Is 15 Months Old. He Was Born

Hello. My son is 15 months old. He was born 6 weeks early due to preeclampsia. From the middle to the bottom of his spine curves outward when he's in the sitting position. He has started walking but only along things he can hold onto. He also will not put a lot of pressure on his right side. Won't roll onto the right side and when he crawls he won't put his right knee on the ground. His dr doesn't seem concerned but I am. Thank you for any advice.