There is no specific age, when you feel that your son can pronounce all the letter correctly you can start with him, Setting a good model, being patient, making dua, and good environment are things that may help.
May Allah make him among His righteous slaves.

I am blessed with a son Alhamdulilah…I wish to make him a Quran hafiz and I want to know which is the best age to begin with n what are the things to b done so that the child is righteous in every way