Issues with sore legs when walking….

My son is 11 and is having issues for a while now when walking. He walks around and after maybe 20/30 mins or less even he starts saying his legs hurt really bad. Hes been to physical therapy which seemed to work and we do all the same things at home and he works out 3/4 times a week. Has anyone else experienced this? I am bringing it back up to the pediatrician tomorrow but wondering if anyone else’s kids have even through this.