My dad taught me to shoot on a .22 bolt action when I was 7 or 8ish if I remember correctly. I didn't get to try out someone's Ruger MkII until much later.
I'm kinda leaning towards rifle first, pistol second, but that's just because that's how I went through it. Plus with a pistol you have to teach the stance and how where to put your hands whereas with a rifle it's easy-butt in shoulder, arm at a comfortable distance, hold your breath for the shot. But honestly, I really don't think you can go wrong either way. If you decide to go with a pistol make sure you do it right and don't put him too far away from the target. Start him out close to let him build up his confidence. Once it seems like he's having fun pull him back and really get into it.