Baby Brother and walking?

My brother was born at around 28 weeks gestation. He was a micro preemie. He is now 13 months old and VERY tiny. He hasnt been diagnosed but he cant cruise or walk. I will try holding his hands and walking but he refuses to walk forward because his knees truly will not let him walk forward. he is getting physical therapy but what else can we do? They cant help much because his knees are so bad. We have no idea what condition it could be nor does the doctor. Any ideas?