mr scout wrote:Several
1. Buy new aircraft your sick of the old clunker anyway
2. Put on a EZ-Flap handle where the yoke is
3. Get new smaller child and start over
4.. Kick current child out, tell him to come back anytime so that he wont
5. Get girlfriend take her flying and leave kids at home

You guys crack me up... but the pre-WW2 Taylorcraft actually had a 'stick" option that looked surprisingly like the previously stated suggestion of a vertical pistol grip, and very much like a certain flap handle extension

But none of those ideas is actually the right way to go IMHO.
It would seem that you could substitute a different yoke on the right side that is smaller, or modify your existing yoke. Since it is LIKELY that the Stinson was certified with the left yoke required and the right yoke optional... you can make a common sense case that (as long as the new right side yoke doesn't interfere with anything) you are no less safe than you would be with the original certified configuration. Now that's common sense, and you may or may not get someone on the other end that wants to do it on a common sense level.
All that being said, the REAL way to go about this smoothly is to have a DAR (Designated Airworthiness Representative) look at your proposed change and accept it on behalf of the government. In this case, you substitute a couple or three hundred dollars in fees for the designee for a couple or three thousand dollars worth of aggravation and delay. And your son gets to fly the Stinson much sooner.
First, find a Cessna, Piper, or whatever yoke that fits in there with your son and that he can fly the airplane with. Make it an airplane control yoke, so you're starting off with an airplane certified part. Like Zane and others have said, turn it upside down, or cut the bottom off it... whatever is needed. Once you're sure it works and clears his knees, then make drawings and measurements. The key is to show the DAR in clear photos and unquestionable measurements that the new yoke does not interfere with any control, upholstery, panel, etc. so it cannot cause any impediment to control of the aircraft. That will be his one and only legitimate concern. Then photograph it with your son in the seat, with both the original Stinson yoke "less safe configuration" and your proposed "new" yoke safer configuration.
If you find the right designee this will be a 30 minute approval process.