I think it can be different for everyone. I know a girl who didn’t start until 15 and got checked out and she was fine. I think it’s more rare though. If you’re worried, have him see his paediatrician who can maybe do a pituitary gland check just in case.
Puberty can start from 10yo to 15yo in boys remember the first signs of puberty are penis & testicle growth then pubic hair. Ejaculating about 1 year after that. What signs puberty does your son have ? And when did he start ?