is unlikely to be persistent for a long time.
Its likely that they are Mongolian spots- which are asymptomatic bluish discolorations, seen in many kids. They appear at birth or within few weeks of birth. They slowly fade off. Usually disappear by 3 to 4 yrs of age. Rarely they persist for life.
You may get it confirmed with your Pediatrician
Hope my answer helps you.
Wish your son good health.