Not all 15’s are equal.
Some are already young adults and some are still large children. No offense but he simply don’t all mature at the same rate.
Flight training (powered) can be intimidating and frustrating and very costly for someone who can’t find his shoes in the morning.
Glider flying is much more a social event with a lower threshold and likely more people in their teenage years.
As stated above, if gliding can’t hold his interest then certainly powered flight won’t either.