The best thing to do is a professional fitting. SEE what the difference is his numbers when using different types of shafts. Keep switching the shafts out and compare. Hit 5 shots with each shaft go through the rotation a few times and you will see the results.

If he has a high ball flight, carries a 7 iron 155 - 170 yards (every time) and has a quicker tempo he is fine to switch to stiff shafts. Driver swing speed for stiff is recommended 90-105mph. His 7 iron swing speed will be around 85-90mph. At 15 might as well start him now since he will be there soon enough. If his 7 iron is still in the 140-155 carry range he is better in a regular flex. A shaft that is too stiff will hit lower and leave shots to the right. There are also many other varibles with shafts besides flex - weight, kick points, material etc.

From what I've read and been told by several fitters, tempo and the transition into the down swing is as much, if not more important than just club head speed.

Mr ONeill is, imo, absolutely correct. I don't think you can make any judgements on shaft stiffness by ball flight or off-line shots. When Karsten Solheim started png, he had True Temper make a lighter weight stiff shaft that he thought appropriate for all golfers. Men, women, kids, seniors...., he believed in one shaft fitting all. John Daly could probably play a regular shaft while Doug Sanders would have needed something extra stiff. The only way to tell the appropriate shaft is to hit a lot of different shafts. And see where the ball goes...

Have been played stiff shafts forever. Went for a fitting for the new T series recently. Was playing the AP1 irons and could on a good day hit my 7 iron around 150 yards. Considering I am 73 years old, no bad, I guess. Wanted more consistency and accuracy. Went to see my fitter, who is a Titleist Product Specialist is our area and he put me in the new T300 irons with the TRUE TEMPER AMT RED R300 shaft and it has made a big difference. When we did the fitting we tried all shafts including graphite and with these shafts we were able to consistently get 160 with this set up. Trackman showed us the way and could see the flight I was seeking. High and over the flag. My driver swing speed has been tracked at 93 mph with ball spin of 1800-2200. I always get fitted and would recommend that ANYONE looking for new clubs do the same.

*Tempo and transition into the downswing creates the club head speed. That is what my instructor work on at least once a week.

I’ve been playing regular shafts for three years just went to an iron fitting even though I just bought new irons to find out my swing speed and ball speed and carry carry. I found out that my swing speed with a seven iron is 99 mph and carries 192 average, what stiffness would you recommend?

You should be more focused on the shaft characteristics and how it fits your swing. What one manufacturer considers stiff could be the same as another's regular. Too many different profiles and weights to just focus on the flex. Swing speed only references how fast the club is moving at a certain point in the swing. True that more swing speed equals more distance, but what good is it when you can't hit your target. The most important aspect of a good fitting is the shaft. It takes only a couple swings to determine which clubhead fits you, but another 30 minutes to determine the proper shaft weight, length, profile, flex that can make that clubhead work for you.

…The beauty of getting fit. Besides club-head speed at impact, the type of transition during your swing is also a factor. So there is no magic calculator to say which flex you need based on speed alone.
If you had a fitting with someone watching you, your dispersion pattern would have revealed if your shaft was too soft. You should have had a tendency to pull shots. Unless you adjusted to that by aiming further away.
Note shaft flex is not a standard measurement. It is only meaningful within that one line shafts. One S shaft may be more similar to another line’s X or even R. Kick points and shaft weight are more important than flex in finding the right shaft for you. The flex in that line is more of a fine tuning that shaft to you.