Lots of good suggestions so far.
I'll stress getting him something that fits his hand. Too big or small will make teaching a challenge.
Forget a budget. Buy him a quality firearm. Something that is accurate, reliable and will last. You want him to have fond memories of both the gun and you when he's still shooting it at your age.
I am also in the revolver camp. A S&W Model 34 or 18 would be perfect depending on which fits his hand better. Also perfect to learn good DA shooting.
Be careful of semi-autos. Most of the small ones really aren't that accurate and triggers are often heavy. While I love my Walther PPK/S .22, the trigger is far to heavy for a beginner to shoot well.
If it fits him, I love the Browning Buckmark. Wonderful pistol, extremely accurate, great trigger and very reasonably priced.