M.M.
You will know when the time comes whether he is ready or not. I have one daugher who we put in at the "right" time and she ended up skipping 2nd. By 6th grade she was completely bored and could have skipped 7th. She is now a sophomore taking senior level clsses.
She started feeling the age difference last year as she was 13 in classes where the bulk of the students were 18.
I have another child who went ot Kindergarten reading and adding but I have kept this one at the grade appropriate for her. I also held her back a year from kindergarten even though she made the cutoff by two weeks.