I just looked at scouting.org and the requirements for joining Boy Scouts are:
Boy Scouting, one of the traditional membership divisions of the BSA, is available to boys who have earned the Arrow of Light Award and are at least 10 years old or have completed the fifth grade and are at least 10, or who are 11, but not yet 18 years old.
That is right from national's website so 9 is not old enough no matter how you look at it.
Besides I don't see how a 9 year old could get the AOL. You have to be active in your Webelos den for 6 months since completing 4 grade( or turning 10). At 9 he should be in 3rd or 4th grade depending on when his birthday is.