During my 20 odd years of coaching I was presented with a huge number of children, ladies and young adults and a whole catalogue of inappropriate guns!
I appreciate the difficulty of finding a gun for a child of only 9 but I think many parents expect their pride and joy to hit the ground running and start shooting game and clays when IMHO they are far too young.
If I had a child of this age I would be offering him only the briefest experience of shooting with a gun and target that he can not fail to hit. Less is more at these early stages and the thought of putting him in a duck blind at this age with a gun would be an anathema to me.
Only when he was physically strong enough to handle a 'real' gun for considerable periods of time without undue fatigue would I consider introducing him to situations where he could fail yet continue to work on the target until he achieved consistent success.
At what age this all takes place is for the parent/guardian to decide but I beg you, don't rush into it. I have seen keen youngster's confidence and enthusiasm utterly destroyed by a parent's over-confidence and -enthusiasm.
The only other piece of advice is to use as big a gauge gun as can be accommodated. 410's are a blight on the inexperienced, 28's are only really suitable for experienced shots or a brief taster, 20's are better but a compromise for the learner, 16's are pretty good but the 12 rules as king.
One of my best lady clients @ about 60 yoa stood about 4' nothing in her socks, built like a straw, weighed less than 90lb, suffered from various ailments but shot a 28"/12g Miroku like a demon! She had started with a 28g but had always struggled. The 12g was a revelation to her. Sure, she got tired but she cracked 75-85% of her targets and loved it.
My other piece of advice is not to impose our love of S/S's on the learner. Don't misunderstand me, I adore S/S's and would never shoot with anything else (and I have a cabinet full of O/U's and semi-auto's to choose from) but for the beginner I strongly believe that an single site plane helps enormously for them to get to grips with targets.
Once they are competent shots with confidence in their ability move on to a nice S/S but start with a O/U or S/A.
I hope this advice is taken in the way it was offered, with hope for a wonderful life time experience in shooting.