First of all to address the real question of the post, I'd say a Ruger Mark II would be great... he'd have to learn two handed stances first until he is big enough to work one handed but nothing wrong with that. Other good ones include the P22, which is smaller than the RUger, grip wise, and alot lighter. Unfortunately, even with the longer 5" barrel and compensator, it kicks. Put some clay putty in the brrel extension area to give it more weight. The S&W 41 is at teh other end, being heavier than the Ruger but its solid it nearly indestructable as far as being behind on maintenance. All have easy tear downs. Clearing the RUger is a pain though, some I'd be more inclined with teh P22 or S&W, since they more resemble modern autoloaders.
Now, what's the bull**** about 7 year olds shouldn't be shooting pistols? What kind of bullcrap is that? I say if your son is disciplined enough by your judgement to work with a pistol, then by God, teach teh youngster. You'll never know when it might come back to save you. (see American Handgunner March/April '03)
I was shooting pistols and rifles when I was 7 and 8 years old. By 12, my father and I were the top doubles team on Maelstrom AFB for the duration of the time we lived there. I am currently a member of my school's pistol team and only the team captain is a better shot in international pistol competition. In IDPA style shooting, I clean his clock. I didn't get this good by having a father who shyed away from teaching me the basics of firearms, especially autoloaders and rifles, seeing as how those were teh two weapons in the house at the time. When I was 10 and older, my father, then in the Navy, worked long hours and my mother went to night school for college. That left me and my brother alone. He fully expected me to be able to operate his .45 and know how to shoot. I didn't shoot a shotgun until I was 13 and a revolver until I was 16. WHy? We didn't have those weapons and my father instructed me that revolvers, being more simplistic to operate, didn't need much training to understand how to use. Since my mother got her .38spl, I learned and became proficient with its use. The shotgun that my father now owns? Well, I'm a firm believer that if you need practice with a shotgun after mastering the rifle, something is seriously wrong with you.
larryw, teach your son how to shoot an autoloader. Sure, its more challenging and more difficult, but he and you both will reap the benefits of that training later on.