What is the percent of children under 5 falling off a high bed and getting caught?

getting caught in between the mattress and the bed frame? Having a injure to the femur bone. My son is 14 months old and he fell off my mother's high bed trying to climb down and his right leg got stuck between the mattress and the bed frame and he broke his leg. He was also premature at birth 3.5 wks early weighting only 4.13lbs at birth and he only weights 17lbs at 14 months. Could his bones be weaker than a full term baby at his age now?