Honestly, I would steer clear of the 22a. I can see why it's on your list to look at, it was on mine too!
However, there is absolutely NO comparison between the 22a and a Ruger MK. The ruger will win hands down every time.
1. The 22a is NOT dry fire safe--you WILL ding the chamber if you dry fire!
2. Buffer life is very short--I've heard many people say that after 800-1,200 rounds the buffer needs replaced. S&W supplies 2 extras, but for many shooters that won't even last a year!
3. FAT GRIP. It might be a little large, or difficult to "lock in" on this grip for your son. Definitely have him check it out. Feels great if you've got big paws, though.
4. A history of cracked slides. I found at least a dozen instances in the little time I spent researching the 22a. Money of the complaintants also noted S&W offered NO help and required purchase of (expensive!) parts.
5. Many complain that theirs is a picky eater....not reliable with *most* lower end ammo. This seems hit and miss...so maybe you'll get lucky and get a gem.
I've heard a LOT of references to the 22a as a "15,000 rounds pistol" meaning it's expected life is only around 15,000 rounds. That's not a very long life in the rimfire world.
That being said, it is worth noting that you hear a lot more negatives online than you do positives. Those that are happy a relatively quiet, while those with problems are quick to speak up. I'm sure a few guys will come along and sing praises.
If you're on a budget, the Beretta Neos is a great buy...$199 for blued, $250 for stainless. I just picked up the 6" stainless and am VERY pleased with it. Best groups so far have been just over 1" @ 25 rested w/ iron sights and inexpensive federal ammo. Seems very well built, no buffers to wear out, good trigger, and very easy to take apart to clean. Personally, I think this is a "best buy" for the price. Comes with 2 mags, too.
Beyond that, the Walther P22 is a good buy that gets a lot of attention in the younger crowds. They used to be jam-o-matics, but some revisions have been made--SMALL GRIP THOUGH!
Browning Buckmark is a VERY good choice. Well made, accurate, reliable--the camper starts around $260
Ruger 22/45 is another great choice...but has a thin grip some don't like. Again, about $250 starting price.