A hybrid approach would be to start your son with some power training, which probably is available locally. The Age limit only applies to soloing, not to receiving training. But especially if he got 15-20 hours in a tailwheel airplane (tri-gear would be ok), would get him to where he was about ready to solo and he would develop most of the skills needed to fly a glider.

I respectfully disagree. When people ask me the difference in glider vs. power training, I tell them it is like learning to ride a bicycle vs. starting with a motorcycle.