Talk to Pediatrician on Bones Weakness Under Weight

My 5.7 years old son facing legs pain at the time of sleeping. We did his Xray and the Xray report tell his bones age is less then his current age. Like hi current age is 5.7 year and bones ages is 3.4 year. In the month of May 2019 his height was 100 cm and last month his weight was 16kg.