I think we see the problem differently. I see a kid who's grossly overweight as the problem. When the guys from his troop come back with pictures and stories of zip lines, maybe that'll light the fire under his tail to get in shape.
As for Scouts not being tough, I think many of them would surprise you. They're not all the most athletic kids which may be why some have latched onto Scouting (not being involved in varsity sports has given them time to pursue Scouting), but they are tough. My wife likes to joke that when the bomb hits, it's the Boy Scouts and the cockroaches who will survive.
Zip lines are only for tough kids? Really? Then you’ll need to explain why they are tourists attractions all over the world and plenty of older, fat, all kinds of out of shape people have enjoyed them.
Again, the Scouts are for developing children into responsible young adults, not for only taking the best of breed-the easy and lazy way out for the adults.
Again, the weight is the problem. Boy Scouting would be a great program for an overweight individual. It's all about learning skills, optimizing self, and goal setting and achieving.
You're right, seeing others rock climb, zip line is exactly the kind of motivation that I think scouting could provide.
Self discipline and self control are part of the Boy Scout training. Good stuff for someone who needs some motivation.
I know Boy Scouts are tough! Budding Eagle scout in my family and your wife is correct, Boy Scouts will be thriving if it hits the fan!