IMHO when to teach a child how to shoot more depends on their maturity level than a flat age. There are some 4 and 5 year olds who listen well to instruction and can handle it and there are some 35 year old men who are unable to contain themselves and handle a gun responsibly.
Completely depends on the person.
Personally my father started teaching me on BB guns at 4 years old with a BB gun that I got for Christmas, after 6 months of that I progressed to a single shot Winchester .22 my father had and then after about 6 months of that and I got a Ruger 10/22 for the next XMas.
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Couple years went by of me taking my .22 rifle out every time we went out shooting and I progressed on to shooting my fathers Swenson 1911 .45 ACP. I developed a nasty flinch after several shooting trips and my father brought out his BHP and a 1911 in .38 Super with some really light loads that he worked up and I stayed on those two pistols plus my Ruger 10/22 for years and then eventually went to centerfire rifles. Started out on a M1 Carbine and a Mauser in .257 Roberts and ended up on a M1 Garand by 12.
Been helping my sister with my niece, we got her a Crickett single shot .22LR. Those are pretty good and might be the ticket for your boy. They're made for small shooters and are a little over a hundred bucks (IIRC I think they were $129 before tax). Here's my niece with her Crickett .22 (pink of course).
That would be my vote for your son if his maturity level is where it should be to handle firearms with direct supervision with you or another adult right at his elbow.
http://www.crickett.com/crickett_22_LR.php
If you or another responsible adult is not at his elbow to take over bad things can happen. The first time my father left my side to go get something out of the truck a bee came along, I wobbled that pistol all over the place trying to get rid of it (while pointing the pistol at my father with the hammer back and my finger on the trigger) and ended up pitching his 1911 right into the sand. I thought he was going to kill me, but he said that it was his fault for leaving me on my own and he was right.