The Ruger is a fine choice for a training weapon.
While most suggest a shotgun for home defense, I have always preferred a handgun. The best gun for home defense is one that you can use in a high tension situation. It should be one without a lot of bells and whistles and one that you've practiced with until its operation becomes second nature. My suggestion to someone who is not going to spend a lot of time at the range is a .357 six-gun. Easy to use and decent stopping power. After that is a double-action semi-auto in 9mm. (9mm isn't the best stopping round, but the recoil is controlable and also easy to shoot) I do agree that compared to a Mini-14, the shotgun is a better home defense choice. If buying a shotgun, get a pump 12ga. Mossburg, Winchester, or Remington are all great choices.
However, why not get both?

Also, the Ruger .22 pistol is a great way to teach him how to shoot a handgun.
Other pieces of advice:
- Take him to a NRA or Game and Fish hunter safety course
- Make him learn the two most important rules of gun safety
1. The gun is always loaded. Don't ever assume someone else has unloaded it. And if you set it down and take your eyes off it with other people around, check it again when you pick it up.
2. Never point it at anything you don't want to destroy. This includes your buddy, the dog and the tv.
The beauty of these two rules is that if he breaks one, the other covers him.
I grew up in a house where my father collected military firearms. He owned over 100 guns. Some of my best childhood memories are of us out in his gunroom for hours, talking, working, or just hanging out together. However he drilled those two rules into my brain. He would even place blanks in the guns he knew I liked to mess around with just to see if I would pull the trigger on one without checking it first (I never did btw). As I was growing up, if I wanted to see a gun, all I had to do was ask. He would take the time to get it down, make sure I was treating it safely until he could trust me on my own. This took the mystery out of guns and it made me respect them. To me guns where a part of life. I believe that this also made me a more responsible person.
Good luck with your son.