I've spent most of my life dreaming of owning left-handed guns, both long and short, only to finally realize that at this point in time, I wouldn't know how to handle one if I did have it. I am so used to handling right-handed guns as a lefty, that a left-handed gun would just confuse me. I actually think I can handle a right-handed bolt action rifle quicker than most right-handed people can. The only time it's a real issue is with right-side ejecting bullpups.
If you really want to get him a left-handed rifle, there are several to choose from, but I don't have any personal experience with any of them. A quick internet search will turn up several viable options.
You might as well go ahead and get your son used to right-handed guns so he will be able to shoot anything, anywhere he goes. As he learns how to operate them correctly "wrong-handed", he will reach a point where he doesn't even notice it anymore. I don't.
Then again, maybe I don't know what I'm missing; but it's not looking for backwards parts when something breaks.