PVTEXAN wrote: Per your request, here is the series of poles that my son has jumped on this year:
14' - 160 - 19.2 flex
14' - 165 - 17.1 flex
14' - 170 - 16.7 flex
14.5' - 165 - 18.8 flex
14.5' - 170 - 16.7 flex
The 15 footer is a 155 with a flex of 16-17.
From JJ"s chart, here's the poles that he's used so far (the 5 thin red ones), and the pole that you're proposing to buy (the thick red ones). Ignore the green highlighting - that was on the table before I added the red lines.
The poles on each row is a similar stiffness. The top row is the lightest, and he bottom row is the stiffest. Each row is slightly heavier than the row above it and slightly lighter than the row below it.

- PVTexan's Poles.JPG (12.92 KiB) Viewed 20263 times
What stands out from this chart is that the 15' 155 isn't the right progression for your son. If he needs something stiffer than a 14-6 170, then he should move up to a 14-6 175 or a 15-0 165.
You haven't mentioned a need to grip higher than 14-6 yet, but since your son's progressing so quickly thru poles, maybe his grip WILL be pushing the limits on a 14-6. So if that's the case, then a 15-0 165 seems like your best investment.
Keep in mind that these considerations are far more important than your original question - re the FX series.
PVTEXAN wrote: Due to the summer weather here in Texas, the poles tend to become a little softer and easier to plant when you are jumping at a turf stadium where the on-field temp is 110+.
It's a falacy that poles become softer in warm weather. The elasticity properties of fiberglass poles don't change when the temperature changes. What happens is that the VAULTER warms up his muscles better in warmer weather, and then runs, jumps, and swings better ... thus bending the pole more.
Kirk