I think a .357 with .38's in it would be good to start, and when he got comfortable with them he could just load the .357.
I own a ruger mk II and I love it, but im sure your son wants to be able to have something not seen as a wimpy calibur. I also owned a Walther P99 and I agree with gobrob that it is a great pistol for small hands. I was very accurate with it and I kind of wish i never sold it. oh well. thats life.
But if I were you I would consider a nice .357 because of the ability to use 2 different cartrages.