I have never taught guitar (except to myself). Just thought I'd start with that disclaimer.

5 feet is a big nine year old indeed. He's only a little shorter than I was when I started learning on a Gibson L5 archtop, which was huge. (Too huge, actually.) I'd say any full sized guitar smaller than a dreadnaught would be fine - a 00-30 or 00-17, for example.

As you've no doubt gleaned, there are many different approaches to left-hand playing. Some just restring a right-hand guitar backwards. Others learn to play right handed. Still others turn the guitar upside down and play like that. That will be between your son and his teacher.

As for taking lessons together, I can see the other point of view here. Perhaps the key question is whether your son wants to take lessons with you or would rather learn on his own.

Good luck!

Dan Schatz