Hmm...
So when my child is 10, and if he shows the ABILITY and UNDERSTANDING to dive, he will. Obviously starting with snorkeling, and free diving, going forward from there. Why should I hold him back?
I wouldn't worry about it too much, as Rhoneman pointed out, this debate is an ongoing one. You look like you are doing a fine job in encouraging your son to love the water.
If a 10 year old can be certified then you can rest assured that the certifying agencies have not arbitrarily picked this age; to do so would open them up to serious litigation in case of a mishap. There is a method behind the madness.
I have the same criteria in place for my oldest children (three year old twins). They are currently learning to swim, then they will progress to snorkeling, free diving, and if, by the time that they turn ten, they have the ability to safely interact with scuba (which I don't doubt because they have already shown great problem solving abilities) I'll allow them to get certified and to dive in STRICT accordance with the published junior open water guidelines. Actually, I will probably be a little bit more conservative when it comes to the 40 foot max depth requirement.
Are there some children who are not ready to dive at age 10? Absolutely. Are there some that are? You bet. When it comes down to it the parent of the child in question is in the best position to determine if the child is ready to undertake the sport. We could examine this issue from an academic standpoint until our eyes begin to bleed, but the truth of the matter is that each and every child is different, and it is up to the parent to assume responsibility for the well-being of their child.
If my kids were to show me that they were not physically/mentally/emotionally ready to dive I wouldn't let them no matter how old they were.