My son broke his Femur when he was 10. He was about 4'10". He is now 6'2". The main concern was his growth plates. They opted for placing a rod in his leg rather than a cast since it appeared the growth plate at the top of his femur was thru growing. As long as they are not damaged it should not be a najor concern especially since your son is in a cast. The biggest issue we had was he went from 85# to 130# in a matter of months.